NEW YORK, FEBRUARY 16, 1894. 
AN EXERCISE IN GEOLOGY. 
BY G. D. SWEZEY, DOANE COLLEGE, CRETE, NEB. 
A most profitable training to be had from the study of 
geology is found in the interpretation of geological maps 
and sections with a view to reconstructing the geography 
of the continent in various periods of geological time. 
Our text-books usually give the student such recon- 
structions ready made, but it is safe to say that they do 
not mean very much to the average student; he does not 
probably get farther into the matter than to wonder how 
anybody knows that there was an extended land mass in 
the Sierra Nevada region, for instance, during Paleozoic 
time when the geological map shows the region mostly 
* covered with Jura-trias rocks. 
As data for this exercise I compile, from as recent data 
as I have at hand, a geological map of the country and a 
considerable number of geological sections. For a blank 
map I use the map ‘‘ Form C” of the United States 
Weather Bureau, 19 X 24 inches in size. 
The geological mapshould be a simple one, omitting 
many small areas; it is not worth while, for example, to 
show the narrow lines of Cambrian and Carboniferous 
rocks bordering the Rocky Mountains; their presence will 
be disclosed by the geological sections; besides they 
would be rather misleading than otherwise, since they 
seem to imply that the Silurian and Devonian are there 
missing from the series. Nor should the intricacies of 
Appalachian geology be represented. I generally con- 
tent myself with showing a very narrow line each of Cam- 
brian, Silurian and Devonian bordering the Archean area 
of this region on its western side. 
I use one color for each period, as now recognized by 
the United States geologists, omitting, however, the 
Pleistocene or at least the drift deposits. For pigments I 
use the analin dyes, approximating as nearly as convenient 
to the colors adopted by the United States Geological 
Survey, as follows: 
Neocene— Yellow analin tinted slightly with rosin. 
Eocene— Yellow analin. 
Cretaceous—Methyl green shaded with yellow. 
Jura-trias—Methyl green. 
Carboniferous—Blue analin. 
Devonian—Gentian violet darkened with common ink. 
Silurian—Gentian violet. 
Cambrian—Rosin. 
Algonkian—Yellow tinted with rosin. 
Archzan—Bismark brown. 
_ On the same sheet with the map is presented a general- 
ized section across the continent, on the goth parallel, 
showing the superposition of the rocks of the several 
periods, their relative thickness in different basins, their 
folding in mountain regions, their conformity or uncon- 
formity and some of the more extensive faults which the 
section crosses. This section along the 4oth parallel 
happens to be an unusually instructive one, crossing, as it 
does, surface exposures of every formation, except perhaps 
the Algonkian, and revealing the geological history of our 
principal mountain systems; but in addition anumber of 
local sections are needed to make clear the history of 
certain regions, especially where late formations entirely 
conceal earlier ones. I have represented sections across 
the Green Mountains and the basin to the east of them, 
across the Connecticut Valley, through one or more of the 
Great Lakes to show that they are erosion valleys, through 
the Black Hills, the Uinta range, the Texas Archzean and 
Algonkian, the Grand Canon region, etc., etc. These 
sections should be on the same sheet with the map and 
numbered to correspond with lines on the map indicating 
their location. 
Some of these sections must, it is true, be more or less 
hypothetical, but they should not be mere guesses; let the 
guessing be done, if it must, when we come to reconstruct 
the geography of the continent. Portions of the north- 
western and southwestern United States are as yet so in- 
completely known that I do not attempt to include them 
in the map even. 
Finally some lines of off-shore soundings should be drawn 
around the map to indicate where the real borders of the 
continental plateau lhe. 
In the first place each student should make for him- 
self an exact copy of the map and sections. ‘his will not 
be a very laborious task, as a blank map can be placed 
over the other against a window and the division lines 
copied through. By the process of drawing and coloring 
the map the student will get a better acquaintance with 
it than he could in any other way. 
The classes are now prepared to trace the growth of the 
continent from period to period. Let them make at least 
one map showing the land and water for each period. 
Shade the land one color and leave the oceans and sub- 
merged portions of the continent blank; where the coast 
lines can be located with reasonable confidence, indicate 
them by the water-lines ordinarily used on maps; where 
they are quite hypothetical use dotted lines or some similar 
device; but let every student make /us map, even though 
in places it must be largely conjectural. The smaller 
weather bureau map 9 X 12 inches will perhaps be better 
suited to this purpose. 
I have been very much interested to see how, by this 
process, a geological map from being, to many a student, 
a meaningless patchwork of colors becomes significant 
and intelligible, almost a geological history in itself, in 
which the student can see in imagination not only the 
gradual extension of the continent southward from 
Canada during the earlier periods, but also the sinking 
archipelago in the west with only its higher summits 
finally peering above the engulfing seas. 
I have suggested one map for each period; but there 
are some portions of the geological story so interesting 
on account of their widely changing conditions that 
several intermediate maps are most instructive: this is 
especially true of pre-Cambrian and early Cambrian times 
and also of the passage from Carboniferous through 
