82 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
deposit was made in lacustrine waters, and that such a surrounding 
land surface as that just mentioned existed at the time the deposit was 
made. The study of North American geology has revealed the pres- 
ence, especially in the broad interior region of the continent, of many 
such lake deposits containing remains of the Unionide, the earliest 
one of which that will be referred to being of Triassic age. These 
deposits alternate with marine formations, showing that the conti- 
nent has risen by repeated oscillations of the land surface with rela- 
tion to sea-level, and not by one uniform upward movement extend- 
ing through successive geological ages. The aggregate gain of 
these oscillatory movements is the present elevation of the continent. 
The first land that appeared above sea-level was drained of its 
surface waters by brooks; and as the land increased in extent the 
waters of the brooks increased in volume and became rivers. The 
unequal elevation of continental land occasionally caused broad de- 
pressions of its surface, which filled with drainage water and 
became lakes. Each lake, together with its outlet and inlets, became 
stocked with a fresh water fauna which was derived from some pre- 
existing fauna. Because all existing lakes and rivers contain mol- 
luscan life, and because all lacustrine deposits contain remains of 
such life, it is necessarily inferred that formerly existing lakes and 
rivers were stocked in like manner. 
Lakes are parts of unfinished river systems. The deepening of 
the outlet portion of such a river system by its running water, aided 
by sedimentation in the still water, drains the lake and finishes the 
river system. For example, referring to existing rivers, all parts of 
the Mississippi system are finished except the slight expansion called 
Lake Pepin. The St. Lawrence system is very far from finished 
because of the great, and many smaller, lakes that still remain in 
both its principal and subordinate courses. 
As a rule, abrupt land elevations, including mountain ranges, 
which have resulted from foldings and other displacements of the 
earth’s crust, have risen so slowly from previously plain regions, 
that the rivers which were already established there were not only 
not thereby obliterated, but usually they were not even materially 
deflected from their courses. By the corrasive action of its running 
water and the detritus which it carried by its flow, each stream 
abraded and carried away the earth-material, even including solid 
rock, as it slowly rose beneath its channel. Some of the now ex- 
isting rivers have thus made deep cafions with precipitous sides, 
through the rocky strata of elevated regions, and some have even 
cut their way through mountain ranges. The cafion sides represent 
