statue. The design is as yet hardly perceptible in the rough cut block,
from which the chisel strikes off large chips at every blow; but on
looking closer the antiquary discovers that the block itself is an old
torso, broken and weather beaten, and at once his imagination runs back
through its earlier history. This is not the first time that the marble
has lain on a sculptor's table, and suffered the strong blows of the
first rough shaping. Long ago it was chipped and cut and polished into
shape, and perhaps even set up in its completed form in some garden,
but then it was neglected and badly used, thrown over and broken, till
its perfect shape was lost, and it was sold for nothing more than a
marble block, to be carved over again if the sculptor sees fit. Now it
just beginning its second career. We may find many parallels to this
story in the land about us, when we study its history through its form.
The sequence of events and consequently of forms is so apparent here
that no one could have difficulty in interpreting history from form,
and it shall come to be the same in geography. The gorge of the
Wissahickon through the highland northwest of Philadelphia can have no
other interpretation than one that likens it to the first quick work of
the sculptor on the old torso.
An essential as well as an advantage in this extension of the study of
geography will be the definition of types and terms, both chosen in
accordance with a rational and if possible a natural system of
classification. Types and terms are both already introduced into
geographic study, for its very elements present them to the beginner in
a simple and rather vague way: mountains are high and rough; lakes are
bodies of standing water, and so on. It is to such types and terms as
these that every scholar must continually return as he reads accounts
of the world, and it is to be regretted that the types are yet so
poorly chosen and so imperfectly illustrated, and that the terms are so
few and so insufficient. Physical geography is particularly deficient
in these respects, and needs to be greatly modified in the light of the
modern advance of topography. General accounts of continental
homologies of course have their interest and their value, but they are
of the kind that would associate whales with fishes and bats with
birds. The kind of reform that is needed here may be perceived from
that which has overtaken the biological sciences. The better teaching
of these subjects lays representative forms before the student and
requires him to examine their parts minutely. The importance of the
parts is not judged merely by their size, but by their significance
also. From a real knowledge of these few types and their life history
it is easy to advance in school days or afterwards to a rational
understanding of a great number of forms. Few students ever go so far
in school as to study the forests of North America or the fauna of
South America. It is sufficient for them to gain a fair acquaintance
with a good number of the type forms that make up these totals. It is
quite time that geography should as far as possible be studied in the
same way. No school boy can gain a comprehensive idea of the structure
of a continent until he knows minutely the individual parts of which
continents are composed. No explorer can perceive the full meaning of
the country he traverses, or record his observations so that they can
be read intelligently by others until he is fully conversant with the
features of geographic types and with the changes in their expression
as they grow old. Both scholar and explorer should be trained in the
examination and description of geographic types, not necessarily copies
of actual places, before attempting to study the physical features of a
country composed of a large number of geographic individuals. When thus
prepared, geography will not only serve in geologic investigation, it
will prosper in its proper field as well.
Geographic description will become more and more definite as the
observer has more and better type forms to which he may liken those
that he finds in his explorations, and the reader, taught from the same
types, will gather an intelligent appreciation of the observer's
meaning. Take the region north of Philadelphia above referred to.
Having grown up upon it, I called it a hilly country, in accordance
with the geographic lessons of my school days, and continued to do so
for twenty years or more, until on opening my eyes its real form was
perceived. It is a surface worn down nearly to a former base level but
now diversified by ramifying valleys, cut into the old base level in
consequence of a subsequent but not very ancient elevation of a
moderate amount. Maturity is not yet reached in the present cycle of
development, for there is still much of the old base level surface
remaining, into which the valleys are gnawing their head ravines and
thus increasing the topographic differentiation. Perhaps not more than
a sixth of the total mass above present base level is yet consumed. To
say that a country is hilly gives so wide a range to the imagination
that no correct conception of it can be gained, but I venture to think
that one who understands the terms used can derive a very definite and
accurate conception from the statement that a certain country is an
old, almost completed base level, raised from one to three hundred
feet, and well advanced toward maturity in its present cycle of change.
It is from geographic methods thus conceived that geologic
investigation will gain assistance. As the subject is properly
developed it will form an indispensable part of the education of every
explorer, topographer and geologist; and in its simpler chapters it
will penetrate the schools. There is no other subject in which there is
greater disproportion between the instruction, as commonly carried on,
and the opportunity for application in after life. The intelligent part
of the world is travelling from place to place to an extent that our
fathers could not have believed possible, and yet not one person in ten
thousand has any geographic instruction that enables him to see more
than that a river is large or small, or that a hill is high or low. The
meaning of geography is as much a sealed book to the person of ordinary
intelligence and education as the meaning of a great cathedral would be
to a backwoodsman, and yet no cathedral can be more suggestive of past
history in its many architectural forms than is the land about us, with
its innumerable and marvelously significant geographic forms. It makes
one grieve to think of the opportunity for mental enjoyment that is
lost because of the failure of education in this respect.
It may be asked perhaps how can one be trained in geographic types,
seeing that it is impossible for schools to travel where the types
occur. This is surely a great and inherent difficulty, but it may be
lessened if it cannot be overcome. Good illustrations are becoming more
and more common by means of dry plate photography; maps are improving
in number and quality; but the most important means of teaching will be
found in models. No maps, illustrations or descriptions can give as
clear an idea of relief as can be obtained from a well-made model, and
with a set of models, fifty or sixty in number, the more important
types and their changes with age can be clearly understood. Maps,
illustrations and descriptions supplement the models. The maps should
be contoured, for in no other way can the quantitative values be
perceived that are essential to good study. The illustrations should be
of actual scenes; or, if designs, they should be designed by a
geographic artist. The descriptions should wherever possible be taken
from original sources, in which the narrator tells what he saw himself.
It is, to be sure, not always possible to know what kind of a form he
describes, owing to lack of technical terms, but many useful examples
can be found that may then be referred to their proper place in the
system of geographic classification that is adopted.
I shall consider only one example in detail to show how far short, as
it seems to me, geography fails of its great opportunity, both as
taught in schools and as applied in after life.
In northeastern Pennsylvania there are several water-falls that leap
over tilted beds of rock. Such falls are known to be of rare
occurrence, and we may therefore inquire into the cause of their rarity
and the significance of their occurrence in the region referred to.
We may first look at the general conditions of the occurrence of
water-falls. They indicate points of sharply contrasted hardness in the
rocks of the stream channel, and they show that the part of the channel
above the fall has not yet been cut down to base level. When the
channel reaches base level there can be no falls. Now it is known from
the general history of rivers that only a short part of their long
lives is spent in cutting their channels down to base level, except in
the case of headwater streams, which retain youthful characteristics
even through the maturity of their main river. Consequently, it is not
likely that at any one time, as now, in the long lives of our many
rivers, we should see many of them in their short-lived youthful phase.
Falls are exceptional and denote immaturity. They endure a little
longer on horizontal beds, which must be cut back perhaps many miles up
stream before the fall disappears, than on tilted beds, which must be
cut down a few thousand feet at most to reduce them to base level.
Falls on tilted beds are therefore of briefer duration than on
horizontal beds, and are at any time proportionately rarer. On the
headwater branches of a river where youthful features such as steep
slope and sudden fall remain after the main river has a well-matured
channel, we sometimes find many water-falls, as in the still young
branches of the old Ohio. These are like young twigs on an old tree.
But even here the rocks are horizontal, and not tilted as in the cases
under consideration.
The falls of such headwater streams must persist until the plateau is
cut away, for the cap rocks over which the streams leap being
horizontal cannot be smoothed down till the whole plateau is cut
through. They are long-lived features. Moreover every one of the
innumerable branch streams must on its way down from the uplands fall
over the outcropping edges of all the hard beds. The falls will
therefore be common as well as long-lived features. Their frequent
occurrence confirms the correctness of this generalization. On the
other hand, in regions of tilted rocks, the hard beds are avoided by
the streams, which select the softer strata for their valleys. The hard
beds soon stand up as ridges or divides, across which only the large
streams can maintain their courses, and these are the very ones that
soon cut down any fall that may appear in their early stages. Falls on
tilted rocks are therefore rare not only because of their brief
duration, but also because tilted rocks are crossed by few streams,
except the large ones, which soon cut away their falls.
The foregoing considerations show clearly enough that falls like those
of northeastern Pennsylvania are rare, and we have now to consider why
they should be prevalent in the region in question. The Appalachians
contain many water-gaps cut down on tilted beds, every one of which may
have been the site of a fall for a relatively brief period of river
immaturity, but this brief period is now left far in the past. The
streams show many signs of maturity: their slope is gentle and their
valleys are wide open from Alabama to Pennsylvania, but in the
northeastern corner of the latter State we find a group of streams that
leap over high benches into narrow gorges, and the benches are held up
by tilted rocks. Manifestly the streams have in some way been lately
rejuvenated; they have been, in part of their courses at least, thrown
back into a condition of immaturity, at a time not long past, and, as
has so well been shown by White, the cause of this is the obstruction
of their old channels by irregular deposits of glacial drift. Here
first in the whole length of the Allegheny section of the Appalachians
we find an exceptional condition of stream life, and here also we come
into a region lately glaciated, where heaps of drift have thrown the
streams out of their old tracks. The explanation fits perfectly, and if
it had not been discovered by inductive observation in the field, the
need of it might have been demonstrated deductively. It is a case that
has given me much satisfaction from the promise that it holds out of a
wide usefulness for geography, when its forms are systematically
studied and its principles are broadly applied.
A final word as to terminology. The material common to geography and
geology may be included under the name physiography, as used by Huxley.
It is, I think, a subject that is destined to receive much attention.
Physical geography, as ordinarily defined, does not cover the ground
that it might fairly claim. It is too largely descriptive and
statistical. Geographic evolution, as defined by Geikie, is the general
preparation of existing geography by geologic processes. It does not
consider the general scheme of topographic development or the natural
classification of geographic forms.
It is not easy to change the accepted meaning of a term, and I would
therefore suggest that a new term should be introduced to include the
classification of geographic forms, as advocated here, rather than that
any old and accepted term should be stretched over a new meaning. As
the essential of the study here outlined is the systematic relation of
form to structure, base level and time, the new term might be
Systematic Geography.
THE CLASSIFICATION OF GEOGRAPHIC FORMS BY GENESIS.
BY W. J. MCGEE.
Scientific progress may be measured by advance in the classification of
phenomena. The primitive classification is based on external
appearances, and is a classification by analogies; a higher
classification is based on internal as well as external characters, and
is a classification by homologies; but the ultimate classification
expresses the relations of the phenomena classified to all other known
phenomena, and is commonly a classification by genesis.
The early geologic classification was based chiefly upon simple facts
of observation; but with continued research it is found that the
processes by which the phenomena were produced may be inferred, and,
accordingly, that the phenomena may be grouped as well by the agencies
they represent as by their own characteristics. Thus the empiric or
formal laws of relation give place to philosophic or physical laws
indicating the casual relations of the phenomena, and the final
arrangement becomes genetic, or a classification by processes rather
than products.
The phenomena of geography and geology are identical, save that the
latter science includes the larger series: since the days of Lyell the
geologist has seen in the existing conditions and agencies of the earth
a reflection and expression of the conditions under which and the
agencies by which its development has been effected; the far stretching
vista of geologic history is illuminated only by knowledge of the earth
of to-day; and the stages in geologic development are best interpreted
in terms of geography. So a genetic classification of geologic
phenomena (which is rendered possible and intelligible through
geographic research) will apply equally to geography, whether
observational or of the more philosophic nature which Davis proposes to
call Systematic Geography, and which Powell has called Geomorphology.
Such a classification is here outlined.
* * * * *
The various processes or movements with which the geologist has to deal
fall naturally into two principal and antagonistic categories and five
subordinate categories; and each category, great and small, comprises
two classes of antagonistic processes or movements.
The initial geologic movements (so far as may be inferred from the
present condition of the earth) were distortions or displacements of
the solid or solidifying terrestrial crust, occurring in such manner as
to produce irregularities of surface. These are the movements involved
in mountain growth and in the upheavel of continents. They have been in
operation from the earliest known eons to the present time, and their
tendency is ever to deform the geoid and produce irregularity of the
terrestrial surface. The movements have been called collectively
"displacement" and "diastrophism," but in the present connection they
may be classed as _diastatic_, or, in the substantive form, as
_deformation_. Recent researches, mainly in this country, have
indicated that certain diastatic movements are the result of
transference of sediment--that areas of loading sink, and areas of
unloading rise; but it is evident that the transference of sediment is
itself due to antecedent diastatic movements by which the loaded areas
were depressed and the unloaded areas elevated; and the entire category
may accordingly be divided into _antecedent_ and _consequent_ diastatic
movements. A partially coincident division may be made into
_epeirogenic_, or continent-making movements (so called by Gilbert),
and _orogenic_, or mountain-making movements. Though there is commonly
and perhaps always a horizontal component in diastatic movement, the
more easily measured component is vertical, and when referred to a
fixed datum (_e.g._ sea level) it is represented by _elevation_ and
_depression_.
The second great category of geologic processes comprehends the erosion
and deposition inaugurated by the initial deformation of the
terrestrial surface. By these processes continents and mountains are
degraded, and adjacent oceans and lakes lined with their debris. They
have been in active operation since the dawn of geologic time, and the
processes individually and combined ever tend to restore the geoid by
obliterating the relief produced by deformation. The general process,
which comprises _degradation_ and _deposition_, may be called
_gradation_.
The first subordinate category of movements is allied to the first
principal category, and comprises, (1) the outflows of lavas, the
formation of dykes, the extravasation of mineral substances in
solution, etc., (2) the consequent particle and mass movements within
the crust of the earth, and (3) the infiltration of minerals in
solution, sublimation, etc.,--in short, the modification of the earth's
exterior directly and indirectly through particle movements induced by
the condition of the interior. These processes have been in operation
throughout geologic time, though they perhaps represent a diminishing
series; they have added materially to the superficial crust of the
earth; and it is fair to suppose that they have modified the geoid not
only by additions to the surface but by corresponding displacements in
their vicinity. The category may be tentatively (but rather improperly)
called _vulcanism_, and the antagonistic classes of movements
constituting it are _extravasation_ and its antithesis. The vibratory
movements of _seismism_ probably result from both deformation and
vulcanism under certain conditions.
The second subordinate category of processes is closely linked with all
of the others. It comprises the various chemic and chemico-mechanical
alterations in constitution and structure of the materials of the
earth's crust. The processes have affected the rocks ever since the
solidification of the planet, though probably in a progressively
diminishing degree; and they have materially (but indirectly rather
than directly) modified the internal constitution and external
configuration of the earth. The processes may be collectively called
_alteration_; and the antagonistic classes into which the category is
divisible are _lithifaction_ and _decomposition_ in their various
phases, or _rock-formation_ and _rock-destruction_.
The third subordinate category of processes, viz: _glaciation_, is
related to the second principal category; but since (1) it is probable
if not actually demonstrable that under certain circumstances glacial
grinding tends to accentuate preëxisting irregularities of surface, and
since (2) it is well known that glacial deposition sometimes gives
great irregularity of surface, it is evident that glaciation is not a
simple process of gradation, but must be clearly distinguished
therefrom. A considerable portion of the earth's surface has been
modified by glaciation during later geologic times. The general process
comprises _glacial construction_ and _glacial destruction_.
There is a fourth subordinate category of processes, which is also
allied to gradation, viz: _wind-action_, which may be made to include
the action of waves and wind-born currents; but since the winds scoop
out basins and heap up dunes, while the waves excavate submerged
purgatories and build bars, it is evident that this category, too, must
be set apart. The processes are only locally important as modifiers of
the land surface of the globe. They comprise constructive action and
destructive action.
There is a final category which is in part allied to alteration but is
in part unique, viz: the chemic, mechanical, and dynamic action of
organic life. Ever since the terrestrial crust become so stable as to
retain a definite record of the stages of world-growth, life has
existed and by its traces has furnished the accepted geologic
chronology: at first the organisms were simple and lowly, and affected
the rocks chemically through their processes of growth and decay, as do
the lower plants and animals of the present; later, certain organisms
contributed largely of their own bodily substance to the growing
strata; and still later, the highest organisms, with man at their head,
have by dynamic action interfered directly with gradation, alteration,
and wind-action, and thus, perhaps, indirectly with the more
deep-seated processes of world growth. The vital forces are too varied
in operation to be conveniently grouped and named.
These categories comprise the various processes contemplated by the
geologist, and collectively afford an adequate basis for a genetic
classification of geologic science. Their relations are shown in the
accompanying table:
_Classification of Geologic Processes_.
| 1.--Deformation. | Antecedent<Epeirogenic. | Elevation.
Principal | | Consequent>Orogenic. | Depression.
Categories. |
| 2.--Gradation. . . . . . . . . . . . . . . | Deposition.
| | Degradation.
| 1.--Vulcanism. | Extravasation.
| | (Antithesis of Extrav.)
|
| 2.--Alteration. | Lithifaction.
| | Decomposition.
|
Subordinate | 3.--Glaciation. | Glacial construction.
Categories. | | Glacial destruction.
|
| 4.--Wind action. | Wind construction.
| | Wind destruction.
|
| 5.--Vital action. | Various constructive and
| | destructive processes.