GEOLOG Y. 107 
Britain, Canada, parts of Europe and the United States, 
the geology of the accessible portion, even, of the earth is 
still very little known, large parts of Africa, Asia, and 
South America being as yet, comparatively speaking, un- 
explored. Civilization, through its railroad-building, tunnel- 
ing, canal-making, and mining operations, furnishes a large 
amount of the material on which the Geologist bases his 
science. Through agencies of this kind, rocks have been ex- 
posed which otherwise would have perhaps remained forever 
concealed from view. Through the excavating incidental 
.to mining and tunneling, there have been discovered the 
remains of plants and animals long since extinct, the relics 
of an indefinitely remote past, the existence of which had 
not been previously even dreamed of. The detritus brought 
down by rivers, and the consequent filling up of their 
mouths, as seen in the deltas of the Mississippi and the Nile, 
with the preservation in the mud, etc. of the coral stones, 
shells, skeletons of fish, etc. which lived and died in the 
vicinity, give one a good idea of the manner in which 
petrified organic remains or fossils may have been preserved 
in the rocks. While in certain rocks of this kind the fossils 
are found in great profusion and in a very excellent state 
of preservation, in others very few occur, or only a frag- 
ment may have escaped destruction. This is often, how- 
ever, so characteristic that the “comparative anatomist 
can reconstruct the whole skeleton from a single bone, 
a knowledge of the correlation of forms enabling the 
osteologist to infer from the structure of the foot the 
nature of the jaws, teeth, etc., of the extinct animal. Many 
such inferences might be mentioned, all of which, while 
commanding praise as illustrating the osteological knowl- 
edge of the anatomist, scarcely merit the astonishment 
which they invariably excite. While many rocks seem 
to have experienced but little disturbance since their origi- 
nal deposition, the different layers or strata of which they 
