INSTRUCTION 
IN 
Geological Investigation 
BY 
WILLIAM MORRIS DAVIS. 
Revised from a paper read before the Association of American Naturalists 
in Philadelphia, December, 1886. 
Reprinted from the American Naturalist for September, 1887. INSTRUCTION 
IN 
GEOLOGICAL INVESTIGATION. 
INSTRUCTION in geological investigation is so new a depart- 
ment of teaching that it is little assisted or hampered by tra- 
ditional methods. Its best materials are found in the wide out- 
door laboratory, and as all out-door laboratories are of their own 
local kind, every teacher is required to develop his own methods 
of using them ; and he must count this to his advantage, for it 
prevents him from doing his own work in some other person's 
way. The personality of the teacher must appear here if any- 
where ; and it must be strongly flavored with the local problems 
that spring up around him in new crops every year, and with his 
own methods of attacking them. Instruction in geological in- 
vestigation at any one place cannot therefore hope to reach the 
completeness that may be attained in physics or chemistry, where 
experiments are made to order in-doors, limited rather by the 
funds than by the place of the laboratory. Consequently, one 
of the first aims of the instructor, and of the student too if he 
take a proper share in laying out his own course of work, should 
be to see that his geological studies are pursued under more than 
one teacher and in many more than one place. So distinct is 
this need, and so strong do I feel the limitations that determine 
it, that I shall not presume to talk up to the general title sug- 
gested by the committee in charge of our meeting, but shall at 
once make my statements specific and tell you little more than 
my own plan and practice of instruction in this broad subject. 
Neither plan nor practice has much age to recommend it; both 
vary, to their improvement I trust, from year to year; and it 2 
Instruction in Geological Investigation. 
should be explained at the beginning that my experience does 
not reach to either one of two branches of geological study that 
occupy much attention,-palaeontology and petrography: there 
is enough to begin with in structural and physical geology. 
Investigation begins with learning to see for oneself. The first 
teaching in geological investigation should therefore be made 
early in the course of study, and not postponed so that only ad- 
vanced students and specialists can reach it. College sopho- 
mores, having had a general lecture course on geology in their 
freshman year, are well prepared for the first steps; but they are 
truly first steps that are then taken, for the face of the country 
has no expression to young men who have indeed learned what 
is considered the saving quantity of classics, mathematics or 
history, but who are unpractised in observation; their eyes are 
not yet opened to the sight of the land about them. Sometimes, 
to be sure, a student comes bringing with him the popular im- 
pression that he is something of a geologist because he can give 
the specific names of a number of minerals and fossils, learned 
in a cabinet; and he may be excused for thinking so, for does 
not every geologist meet, during his summer travels, the em- 
barrassing necessity of simulating an interest in the kind atten- 
tions of his host who wishes to exhibit some " curious mineral" 
or " fossil footprint," as if the geological attractions of the neigh- 
borhood centred therein ? The mental philosophy of geology 
is still an unopened book even to intelligent people. Naturally 
enough, therefore, the student, who has imbibed the popular con- 
ception of the study, or who has learned only from books and 
lectures a series of verbal definitions and explanations, comes to 
a full stop on being told that he is to discover for himself and 
from his own observation the correlation and sequence of geo- 
logical phenomena in the district around him. The-fundamental 
difficulty that he feels is the inability to see what he looks at. 
When this difficulty is fairly overcome, the gain that he has made 
is of so great value in his general as well as in his geological 
education that I find in this very fact the warrant for the election 
of the study of field geology by many more young men than the 
few who may intend to follow the subject professionally in later 
years. Let us consider an actual case. The first excursion that 
I commonly take with a class leads us to an old quarry in Somer- 
ville, near the Agassiz Museum in Cambridge, where a large dike, Instruction in Geological Investigation. 
3 
some forty feet wide, cuts across the beds of the Somerville 
slates. There may be ten or twenty students in the party, and 
it should be remembered that they have had a preliminary course 
in elementary geology, in which nearly all the terms that we have 
occasion to use have been defined : they are also provided with 
hammers, compasses, clinometers, note-books, and outline maps 
of the district. On entering the quarry, I select two fragments 
of rock: one exhibits a fine, granular texture, with bands of al- 
ternating color, and is shown as the type of a bedded, stratified, 
or aqueous rock ; the other is of crystalline texture, without 
arrangement in layers, and represents the group of massive, crys- 
talline, or igneous rocks; and without further explanation than 
this, the students are asked to search out the area occupied by 
each rock, the line of contact and the phenomena exhibited along 
it, and to determine the relations of the two and the sequence of 
events in their history. Emphasis is given to the importance of 
personal work, and I take pains to say how much more valuable 
is the ability to determine the facts than the facts themselves. 
For several years it was my practice to point out the quarry- 
structure on the first excursion, thinking it too difficult for 
beginners to discover; but it seems now that that was almost a 
waste of time. It must be borne in mind that the students who 
are set to this work have already had in their preliminary course 
some account of just such facts as they now encounter in the 
field; they do not come to the task unprepared. Moreover, the 
attention of the class is not so well held by explanation from an 
instructor as by exploration for themselves. I therefore throw 
them at once on their difficulties; my own endeavor being rather 
to suggest observations and give encouragement than to answer 
questions. The questions are to be answered by the rocks. 
Even the best students are almost helpless at first,-so little has 
their general education taught them of independent, original ob- 
servation even of a simple kind. One of the commonest state- 
ments is, " I don't know how to go to work," upon which it is 
necessary to repeat that the first step is simply to search out the 
area of the two rocks and the phenomena of their line of con- 
tact. Not knowing how to go to work on such a problem does 
not mean ignorance of geology alone, but inexperience in the 
first requisite of scientific study. But the most characteristic 
feature of the first day's work is the frequency with which the 4 
Instruction in Geological Investigation. 
instructor's authority is appealed to in decision of questions that 
lie open before the class. It is as if the weight of previous 
education were thrown on the side of dulling individual obser- 
vation and judgment. It requires much care to avoid answer- 
ing easy questions and thus defeating the objects of the ex- 
cursion; and it requires more care to escape answering the 
questions without either discouraging or exasperating the ques- 
tioners. A preliminary explanation is advisable, so that all may 
understand that there is nothing like a rebuff intended in the 
rather brusque counter-questions that serve best to direct the 
work where it should go. A student will sometimes come up to 
me, after a very insufficient search for facts on the ground, and, 
presenting a piece of the dike, say, with the idea that he is doing 
his full duty, " Isn't this melaphyr ?" Now, as a matter of fact, 
he is, in this particular case, quite right; at least, so I am assured 
by competent lithologists, and later on he should be told so, but 
not at first. He has perhaps heard the name, melaphyr, asso- 
ciated with dark-colored, fine-grained rocks, and makes a lucky 
venture in using it; but as the object of the work is to train his 
observation, not mine, I throw the burden of proof upon him by 
asking in return, "Why do you think so?" " It looks like it." 
" What is melaphyr ?" This may sound, as I now read it, very 
much like snubbing a praiseworthy inquiry ; but see the result. 
Nine times out of ten the student says, " Oh, melaphyr? Mel- 
aphyr is I don't remember;" and this clears away the false 
knowledge that places names uppermost, and brings us down to 
a solid foundation for good work. No words must be used that 
cannot be defined: no suggestions must be made that cannot be 
justified by actual observation. The question may now be re- 
turned, " If you cannot say that this is melaphyr, what can you 
say about it?" " Well, it is a dike." " Why do you think so ?" 
" It looks like one." " What is a dike ?" " A dike is a mass of 
igneous rock filling a fracture in the country-rock." " Does this 
igneous rock fill such a fracture ?" Again it appears how much 
easier it is to make assertions than to defend them ; it is very 
seldom that a student will on his first endeavor suggest and ap- 
ply the tests that furnish him with answers to such questions as 
these. He must make a second and a third attack before he 
really gets possession of all the facts about the dike,-its width, 
dip, and direction; the detail of its contact phenomena; its tex- Instruction in Geological Investigation. 
5 
ture and its joints; but by persevering attention all these facts 
can be discovered from his own seeing, and then a good lesson 
is learned. A few such lessons correct the habit of using words 
recklessly, of making guesses and of offering unjustifiable sug- 
gestions in explanation of discovered facts. All this is very ele- 
mentary ; but it is the only basis for good work of a higher order. 
The better students take to the work with fine spirit; in some 
cases the opening of their eyes, the awakening of the conscious- 
ness that the work is in their hands and that they are really able 
to undertake it, is an inspiring experience to the instructor,-one 
that serves to carry him over the disappointment occasioned by 
the obdurate youths who insist that they, who have most to 
learn, cannot learn anything from the quarry before them. It is 
indeed a real pleasure to go with an appreciative new-comer to 
an old, familiar field and watch his rapid learning of new facts 
and methods: it is this that saves teaching from becoming mo- 
notonous, and makes it instead a very live work. 
It is not always easy to secure a good conception of the truth 
and actuality of phenomena that are named and talked about 
familiarly enough; for 
example, the intrusion 
of the dike that has just 
been discovered. Some 
little commonplace de- 
tail is then an aid. I 
suggest that a certain 
part of the edge of the 
dike in question be ex- 
amined minutely and 
drawn to scale, and that an explanation be sought for the forms 
thus discovered. A hinged fragment (Fig. i) is soon found, 
but its full significance is seldom seen at first. It is part of a 
"stringer" between two joints which the intruding dike first 
opened (Fig. 2), the other part having been broken off and car- 
ried away out of sight in the further widening of the fracture. 
The penetration of the dense dike-matter into the narrowest 
parts of the crevices thus opened shows how mobile it was under 
the heat and pressure that caused its motion. This specific detail 
carries the observer back to the time of the intrusion, and gives 
reality to it. 
z 6 
Instruction in Geological Investigation. 
A cross-section of the quarry is then asked for, and this brings 
up a very common difficulty arising from lack of practical geomet- 
rical knowledge. The 
same difficulty appears 
in determining the dip 
of the dike; it is as if 
the rocks were to be 
studied only in the two 
' dimensions of length 
and breadth, without 
the third dimension of 
thickness or depth. 
Even so simple a geo- 
metrical matter as strike has generally to be labored over. The 
cross-section of the dike and slate is drawn to estimated scale 
with some care (Fig. 3) 
so as to make a begin- 
ning at least of cor- 
rect under-ground inter- 
pretation, on which so 
much advanced work de- 
pends ; and this leads to 
many new questions: 
*he dike tilt the 
slate ? Did the dike 
cause the variation in the dip of the slate? Did the dike appear 
before or after the slate was tilted ? 
Such questions as these appear utterly obscure and unanswer- 
able to many students: not because they have had no practice 
in geological investigation, but because they are unpractised in 
investigation itself. The relation of observation, hypothesis, and 
verification has not been unfolded to their minds, and it is for this 
reason that I lay so much stress both here and in the field on the 
importance of reasoning, as well as of observation, in geological 
work. Nothing is more important than for the student to per- 
ceive that he may discover and follow the legitimate sequence 
of argument on which geological assertion is based. Practice 
in solving the questions proposed above leads to this desirable 
end. If the dike tilted the slate, the beds should dip away from 
the dike on both sides, as in Fig. 4. They do not, and it is 
Piy 3 Instruction in Geological Investigation. 
7 
therefore concluded that the slate was tilted by some force inde- 
pendent of the dikes. This is soon confirmed by finding that the 
dip of the slate is maintained at a tolerably even angle for a mile 
or more, although intersected in various directions by many 
dikes. .If the dike caused the local irregularity in the dip of the 
slate, the dip should be increased on the south and decreased on 
the north, as in Fig. 5 ; but the observed section in Fig. 3 shows 
Fty £ 
Ft? 5 
a local increase of dip on both sides of the dike, as if the mass 
had been slightly sheared independently of the intrusion. Other 
dikes soon confirm the conclusion that their intrusion did not 
perceptibly affect the dip of the adjoining beds. In deciding the 
relation between the dates of intrusion and tilting, we must first 
rule out any argument based on the present tilted attitude of the 
dike; it may have been vertical originally and then tilted bodily 
with the slate to its present dip, or it may have been intruded at 
its present inclination; therefore no conclusion can be drawn 
from its dip. The only means here available for reaching an 
answer depend on a postulate concerning the joints in the slate. 
The joints may have been made in the slate before it was tilted, 
but they were almost surely made while the tilting was going on. 
Now, the dike is seen to have smooth joint-faces for the greater 
part of its walls; it must therefore have taken advantage of pre- 
existent joints in opening a way for its upward escape, otherwise 
its edge would be ragged. The dike therefore certainly came 
after the joints were made, and probably after the tilting was 
begun. The want of complete decision in this case is not con- 
sidered reason for omitting its consideration. So much of geo- 
logical reasoning leads only to probabilities that the student may 
as well make early acquaintance with this kind of half-way 
demonstration, but great care must be taken to see that he fully 
recognizes the postulates of the argument and does not over- 
draw on their certainty. The desire for final settlement of all 
questions is strong in the student mind; questions that must 8 
Instruction in Geological Investigation. 
be held open for further investigation are not always favorites, 
but I believe they are aids to the habit of mental deliberation. 
In this particular case the doubt is soon lessened, for a little 
later, on the same excursion, a ragged dike is found breaking its 
way unevenly through the slate. The slate is jointed now, and 
traversed by smooth-walled dikes, showing that the irregularity 
of the ragged dike is due not to the impossibility of smooth 
breaks being made in the country-rock, but to their absence at 
the time of its intrusion. Finally, a smooth dike is found inter- 
secting a ragged dike, giving new and safe evidence of the rela- 
tively early and late dates of their respective intrusions; and 
this is emphasized as a good illustration of a double approach to 
a single conclusion. 
One of the most important exercises in connection with the 
field-work is the careful formulation, oral and written, of the 
conclusions reached and the means of reaching them. It helps 
to do away with the idea that a geologist is a collector of speci- 
mens rather than a collector of facts and arguments; it is good 
mental practice, and aids the student in any kind of work he 
may afterwards undertake; it is to most of the class the greatest 
profit that they carry from their excursions. 
One of the most instructive quarries in Somerville is roughly 
drawn in Fig. 6. The attempt is here made to show the visible 
and covered rock but it should be remembered that the 
visible rock is much less distinct in the dusty quarry than in the 
diagram. The tilted slates, S, S, are intersected by a forty foot 
dike, AB, and both are cut by a double dike, CD, of moderate 
thickness. The large dike is soon seen to be horizontally faulted, 
and the question is put, When and where was it dislocated? the 
first answer generally refers the time and place to the newer dike, 
but careful search shows this to be wrong: the great slab of 
country-rock, which here separates the younger dike into two 
parts, is found to match the wall from which it was broken ; it 
consists of slate opposite the slate wall, and of old-dike rock 
opposite the old dike. The newer dike therefore opened its way 
without any lateral shift of its walls. This is confirmed by 
noticing a bend in the dike in the southern part of the quarry, 
the result of a crook in the fracture through which it rose: if 
there had been sheering motion on the fracture, the width of the 
dike at the crook would have been increased or decreased, as in Instruction in Geological Investigation. 
9 
Fig. 7, b, c. The faulting of the large dike is thus shown to be 
independent of and to the west of the fractures occupied by the 
smaller one. Attention is now turned to a slickensided joint- 
FiaC 
face, EF, Fig. 6, on which breccia and fault-scratches are com- 
mon : perhaps the faulting occurred here; if so, how can it be 
demonstrated ? Before looking again at the quarry every student 
is asked to consider two simple alternatives; the large dike 
o 
F{S7 
FlQ 
either was or was not moved on this fault-plane: what would be 
the consequence in either case ? what test may be invented to 
settle the question ? If the dike were not moved on this fault- Instruction in Geological Investigation. 
10 
plane (that is, if the dike were younger than the fault), its margins 
would be in line with each other on both sides of the fault; if 
the dike were moved on the fault-plane, its north and south 
margins would be offset, and both by the same amount. The 
shaded areas marked X, Y, Fig. 8, are thus seen to be of critical 
importance. If X consist of dike-rock and Y of slate, then the 
first alternative is true; if X be of slate and Y of dike-rock, then 
the second alternative is true. Especial attention is called to 
this deductive method of planning out lines of work. It is not 
sufficient simply to go out-doors and look about; the geologist 
must look first, then think, infer, plan, and at last lead his obser- 
vations to the precise spot where they will do the most good; 
not the most good to the theory that may from some accident 
have secured his preference, but the most good in deciding 
critically among all the possibilities of the case. I think the 
history of geology need not be read far before one finds un- 
fortunate examples of the neglect of this method. A fact is 
noted and a suggestion offered : the suggestion fits, and therefore 
it is accepted. But the student must be made to see that such a 
method will not lead to safe conclusions. It is quite possible 
that several processes may bring about the (apparently) same 
result. In such case it is manifestly unsafe to conclude that 
any one process did bring about the result merely because it 
could. Independent evidence must be found to check and con- 
firm a suggested process after its suggestion before any confi- 
dence should be placed in it; further, all other apparently possible 
processes must be ruled out before any one can be accepted as the 
true and actual one. It is not easy to teach this to a class, and 
many will slip through the work without much appreciation of it. 
Sequence of events, as constituting a portion of geological 
history, is a matter of importance. The case just described 
concludes with an illustration of it. The large dike was faulted 
after the intrusion of the smaller one, for on tracing the fault- 
plane to the northern side of the quarry, it is found to intersect 
the western member of the double dike, and brecciate it as well 
as the other rocks. 
The numerous intersecting and faulted dikes on our sea-coast 
north of Boston present unexcelled examples for work of this 
kind: the amygdaloid in Brighton and its banded amygdules 
offer an instructive problem: but the phenomena of the drift are, Instruction in Geological Investigation. 
11 
as a rule, too complicated for local elementary treatment by the 
methods described above. Their observation needs to be sup- 
plemented by many facts drawn from larger fields than our ex- 
cursions cover, and, although very interesting and valuable for 
class-work, they cannot be considered as useful in early teaching. 
Eight excursions of from four to six hours each, four in the fall 
and four in the spring, are thus occupied, and constitute the 
field-work that makes one-third part of our second course in 
geology. They are arranged so as to include as varied examples 
of structure and as varied problems as possible, but their aim is 
discovery and demonstration rather than exposition and illustra- 
tion. They are intended to practise the power of observation, 
to awaken and develop the methods of geological reasoning, 
rather than merely to load the memory or inform the mind. In- 
deed, all this work is like exercise in a gymnasium. A young 
man who practises leaping over a vaulting-horse does not do so 
because he expects to meet the same vaulting-horse in after-life, 
but because he wishes to be able to leap over various obstacles 
outside of the gymnasium. 
The winter season, when out-door work is stopped, can be 
well employed in securing some familiarity with our geological 
literature. Government and State reports with Prime's index 
thereto, the American Journal of Science, and the Royal So- 
ciety's catalogue of scientific papers, with the more important 
journals that it leads to, are our chief materials, but of this I 
will not speak further. 
At the end of the second course with its field-work, the student 
must consider carefully if he wish to go farther into geology. If 
he decide to go on, there are courses in lithology, economic 
geology, and palaeontology before him, but these do not come 
strictly under the title of my essay. There is also a course in 
more advanced field-work, which is more to the present point. 
We find it difficult to give this course as it should be given, on 
account of its necessary conflict with other college duties. It is 
impossible to do much serious work without a whole day in the 
field, and this generally involves the unsatisfactory necessity of 
" cutting" something else. The summer vacation may be utilized, 
but the student is not often well enough prepared to walk alone 
amidst the complication of problems that he meets. The sum- 
mer school is a valued factor here, but this is a rather heavy tax 12 
Instruction in Geological Investigation. 
on the instructor. We make the best of it in one way or another, 
and have succeeded in accomplishing some fairly good work ; but 
in contemplating our moderate successes, and comparing them 
with the more finished products of the biological, chemical, and 
physical laboratories, we have occasion to lament the unwieldy 
character of geological problems. The biologist may cultivate his 
specimens in a jar, slice them nicely in his microtome, and exam- 
ine them with his microscope, all comfortably arranged at a con- 
venient table in a good light. Winter and summer, good weather 
and bad, his work of actual and original observation may go on. 
The geologist has no such good fortune. He must go out of 
town to his work, spending valuable time on the way there and 
back. In our present almost mapless condition he has to play 
topographer to the distraction of his attention, and his problems 
seldom, if ever, can have the conciseness or the singleness that 
may characterize work on animals and plants, and which is so de- 
sirable for the sake of the student. It is difficult to assign sub- 
jects or fields that allow of tolerably complete consideration by a 
student in the fall and spring half-years. The course in field- 
work cannot therefore be described with the definiteness given 
to the more elementary teaching. I can do little more than 
recount its objects. 
The prime object of this advanced work is to teach the 
student not only to see for himself, but to see for himself when 
he is alone. In the first field course, where the instructor 
always accompanies the class, he may direct or suggest a profit- 
able line of work, and hedge the party in from useless searching. 
It is very different when the student goes out most of the time 
alone, the instructor accompanying him only on preliminary and 
occasional later trips. It may be counted a good year's work if 
the student learn in that time to make an accurate, tolerably 
complete, and original report, written and graphic, on two areas 
assigned to him. The want of good topographic base-maps is a 
serious obstacle here; for the present we have to get along as 
well as may be with small-scale road-maps, on which the relief 
of the ground has practically no representation. The topo- 
graphic survey of Massachusetts will, however, soon in part 
remedy this difficulty. The first questions that arise in the field 
concern the division of the country into its kinds; that is, simple 
areal geology. As all of our region is largely drift-covered, it Instruction in Geological Investigation. 
13 
is necessary for good work to map the rocks only where they 
are seen in outcrops, and to classify the drift as well as the 
rocks beneath it. The number of formations and sub-forma- 
tions has to be discussed ; the characteristics of all to be deter- 
mined ; and the boundaries to be marked out. This is so large 
a problem, and is often so complicated, that for the first two or 
three excursions the student may feel discouraged. The best 
scholars learn much by working out their own difficulties, with 
no more aid than is given by answers to admissible questions; 
those with less ability and perseverance need more immediate 
assistance. Repeated visits to the more significant localities are 
advised; for the beginner can hardly hope to reach at once the 
ambition of the most experienced geologist, and see in a first and 
single view not only the raw facts as they then appear, but also 
the critical points that are visible only after reflection. There 
are indeed few fields that do not repay second and third gleanings ; 
if they seem barren after the first harvest, the gleaner had best 
ascribe the barrenness to his own want of skill. 
There are two matters that I find need repeated mention in 
this work. First, the student must as far as possible finish his 
work as he goes, and not depend on later visits for doing what 
he can see on the first; a general reconnoissance is, of course, 
allowable, but unrecorded or sketchy work must not continue 
long. Each day's work must show a portion of the map finished 
as far as the knowledge then possessed will allow; however care- 
fully this is done, there will be enough work for review suggested 
by later discoveries, and quite out of sight at first. The second 
matter is the need of distinguishing explicitly between what is 
seen and what is inferred. Printed maps generally omit this 
distinction on account of the large scale that it requires, but its 
omission on field-maps is inexcusable. Strong colors or full 
lines may be used to indicate well-ascertained facts; faint or 
dotted colors, and broken or dotted lines, aided by interroga- 
tion-points, will signify doubt. Uncolored portions of a final 
map should mean only unvisited country. A piece of country 
carefully surveyed and mapped in this way gives good exer- 
cise, I believe, to the conscience as well as to the observing 
faculty. 
Innumerable problems arise during this work : they are only 
too plentiful. The quality of the student may be finely gauged 14 
Instruction in Geological Investigation. 
by the kind of questions they excite in him; it is needless to add 
that the quality of the instructor may be gauged by his answers! 
As a general rule, the answers should be like guide-boards point- 
ing out the way to the settlement; not like coaches, carrying the 
passive questioner there: the working geologist must learn to go 
afoot, and to find his road rather than be led along it. I may be 
allowed in closing a single illustration from an actual case. 
The problem in hand was concerning the former higher level 
of the sea about Boston. The opinion is commonly entertained 
that there was a post-glacial depression of the land amounting to 
about ninety feet; this being quoted from an old record of sea- 
shells found at that altitude near Winthrop, and confirmed by 
comparable records to the north and south. If such a depression 
occurred, the shore of the sea must have run around the margin 
of the present Boston basin. ■ With this beginning, the student is 
sent into the field, to search the slopes up to one hundred feet 
for water-marks, and to search his mind for argument on the 
whole question. He must be on the alert to perceive any faint 
indication of a rock-cliff, sand-bar, or delta ; he must keep his 
mind wide open to all available hypotheses for their explanation, 
even the most " cranky" being dismissed only for cause, not be- 
cause it is unfashionably dressed ; he must be strictly logical and 
impartial in applying tests to verify his hypotheses, and he must 
give up all that are found wanting. To the first view, the tramp- 
ing over the country may seem the greater part of the field-work, 
but, to one who is able to take the full profit from it, the mental 
exercise is always much greater than the physical. Some 
students find the physical work the harder of the two. 
The discussion will run about as follows: Admitting that the 
Winthrop shells are a true sea-deposit, there ought to be signs 
of an old sea-shore somewhere above the present level, unless 
the shore did not stand at any level long enough to make a 
recognizable mark, or unless the Boston basin was mostly occu- 
pied by glacial ice during the depression. Supposing that a 
shore-mark was made, what would be its appearance ? The 
topography of the present shore-line has to be consulted, and 
there one finds cliffs and benches, bars, marshes, and deltas. 
The first notable characteristic of these varied forms is that they 
stand almost at a single level, the bars being higher than the 
benches; the second is that they are arranged in close accord- Instruction in Geological Investigation. 
15 
ance with the general form of the surrounding country: a sea- 
cliff appears on an exposed headland, not at the end of a 
crooked, narrow inlet; a bar is strung along from a cliff-bench, 
and adjacent bars are sympathetic in their attitude; deltas are 
found only opposite valleys, and of a size proportionate to the 
stream that made them and to its supply with detritus. The dis- 
tinctness of these forms is proportionate to the time during 
which the sea abided at their level; and from this we conclude 
at once that there has been no other post-glacial shore-line of 
anything like as long a life as the present one, along which cliffs 
and bars are so well made. Moreover, all these forms are of 
equivalent size on any given shore-line: if one cliff is formed 
others will be found in corresponding positions, and correlated 
bars and deltas will complete the record. Glacial imitations of 
shore-line topography are also discussed. With these generali- 
zations thought out and talked over, the field-work begins to 
take shape: the student does not wander aimlessly over the area 
assigned to him, but searches intelligently for significant signs of 
water action. On finding a flat, sandy field extending seaward 
from the foot of a clean rocky slope, the hint is followed by 
contouring around the hill-sides; if the walk lead to a more 
sheltered position, look for little deltas where brooks come down 
ravines; if to a more exposed promontory, look for cut cliffs 
and built bars, and so on over the country. The results in this 
particular search were not what is commonly called " satis- 
factory" ; that is, continuous and consistent shore-marks were 
not surely found up to a hundred feet elevation, although 
strongly suspected at sixty feet in some places; but this only 
means that the question is of a larger instead of a smaller order, 
not that it is unprofitable to the student. If the generally ac- 
cepted post-glacial depression and higher stand of the sea at 
Boston is thus discredited by local observation, it must not be 
accepted on the ground of reported depression at not distant 
localities, for they may be similarly discredited, as has lately 
happened on Long Island. The occurrence of high-level shells 
at Winthrop does not alone suffice to prove so large a matter as 
a change of continental level; it only opens a question that 
must be closed by independent evidence, and at present the 
evidence of depression in Southern New England is 
very incomplete. The investigation is therefore highly satis- 16 
Instruction in Geological Investigation. 
factory in leaving us with the watchfulness of uncertainty instead 
of blindfolding our eyes with a definite conclusion. 
I have said but a word about summer schools. They deserve 
more mention, for they are invaluable; they cannot be replaced 
by any term-time instruction. The student is led by them to 
new fields and new problems, so widely different from those of 
his college course, that only the all-embracing subject of geol- 
ogy could bring them under one name. Continuity of work is 
also a great merit of these schools. They do not suffer from the 
distraction of other occupations that ordinary teaching has to 
contend with, and six steady weeks in summer may be made a 
full equivalent of an all-winter's interrupted course. 
It need hardly be said that this course of instruction is not 
planned with a view of teaching the student much of what has 
been learned in geology. That is attended to in other courses by 
means of lectures, reading, and thesis-writing. The work here 
described is designed chiefly to place the student where he may 
gain something of the spirit of investigation. Rocks and struc- 
tures are shown in but moderate variety, and quotation of simple 
fact thus learned is not expected to be of great service afterwards; 
but the facts that have been learned come as directly as possible 
from Nature, whom the student thus finds that even he may 
profitably question. Individual judgment is spurred on to take 
the place of the appeal to authority, and the judgment thus 
grows by exercise even to its own surprise. Thus, with obser- 
vation and argument combined, the student finds himself pos- 
sessed of general methods of work that prepare him to attack 
problems unlike any that he has met in his schooling. Further 
study under new teachers in new fields.may then be followed to 
great advantage, but perhaps the best teacher at this stage is 
practical experience.