Proceedings of the Ohio State Academy of Scieiicc 33 



take many years of study before it becomes establisbed, perhaps 

 much modified, as the accepted theory of glaciation. In the mean- 

 time it largely dominates thinking in this, which has been for a 

 half-century one of the chief fields of geological speculation. 



THE INTERPRETATION OF THE SEDIMENT.\RY ROCKS. 



Twenty-five years ago the petrographic microscope was a 

 new tool in the hands of the geologist, and with its use there 

 came a renewed interest in the older igneous and metamorphic 

 rocks. One could read between the lines that the problems of 

 the sidementary rocks, of their structure, origin and significance, 

 had been largely solved. Yet one can now believe that since that 

 time geological theory has been advanced at least as much by 

 what we have learned from the story of the sedimentary as from 

 the story of the crystalline rocks. And this in two distinct lines. 



Flnviatile, Lacustrine, and Marine Deposits. 



In most discussions of the origin of the shales, sandstones 

 and conglomerates of the older periods, it has been assumed that 

 they are of marine origin, the coarser portion near the shore, the 

 finer farther from the shore line. The similar non-marine 

 Tertiary rocks in our western states the geologists of our western 

 surveys, and after them the texts, have until recently unanimously 

 considered lacustrine, deposits in lakes in some cases thousands of 

 square miles in area, and larger than any existing American 

 lakes. Dana (Manual, 1895) speaks of "the great lakes of the 

 early Tertiary — the Eocene — in the Rocky Summit region" and 

 "the later Tertiary lake basins either to the east or west of the 

 Summit region." And this was done, curiously enough, in spite 

 of the fact that these same workers found in the field and cor- 

 rectly interpreted the extensive deposits of gravel, sand and clay 

 which are today being formed along the base of mountain ranges 

 where streams find their velocity checked as they issue from their 

 mountain-valley courses and are compelled to lay down their load 

 in the form of broad alluvial cones or fans, which merge at a 

 slight distance from the mountain into one piedmont alluvial 

 slope fronting the whole range. In soite also of the fact that in 

 many regions of the world, along the northern base of the Alps 



