408 On American Geological History, 



phate of lime ; and afterwards he found in a niodern Lingula the 

 very same composition, — a further announcement of the harmony 

 between the earliest and latest events in geological history* 



This earliest sandstone, — called in New York the Potsdam 

 sandstone, — and the associated Calciferous sand-rock, mark off the 

 First Period of the Molluscan Age, — the Potsdam Period, as it 

 may be called.f 



Next followed the Trentox Period, — a period of limestones, 

 (the Trenton limestone among them,) equal to the earlier beds in 

 geographical limits, and far more abundant in life, for some beds 

 are literally shells and corals packed down in bulk ; yet the spe- 

 cies were new to the period, the former life having passed away ; 

 and even before the Trenton Period closed, there were three or four 

 epochs of destruction of life followed by new creations. The for- 

 mation of these limestone beds indicates an increase in the depth 

 of the continental seas, — an instance of the oscillation of level to 

 which the earth's crust was almost unceasingly subject through 

 all geological ages until the j)resent. 



After the Trenton Period, another change came over the conti- 

 nent, and clayey rocks or shales were formed in thick deposits in 

 New York, and south, — the Utica slate and Hudson River shales, 

 — while limestones were continued in the West. This is the Hud- 

 son Period ; and with it, the Lovjer Silurian closed.]; 



The seas were then swept of their life again, and an abrupt 

 transition took place both in species and rocks. A conglomerate 

 covered a large part of New York and the States south, its coarse 



* Am. Jour. ScL, [2], xvii, 235, (1854). 



f Through the comparisons of Prof. Hall, it is now well known that the 

 •* Lower Magnesian Limestone" of the west, and a sandstone with which \X 

 alternates, correspond to the Calciferous sandrock of K'ew York. 



\ Prof. Hall, in connection with J. D. Whitney, has recently made the 

 important observation, that the Galena or lead-bearing limestone, which is 

 the upper member of the Trenton group, is separated from the jSTiagara lime- 

 stone in Iowa and Wisconsin by thick strata of Hudson River shales, giving 

 a prolongation to tb ese shales before unsuspected. He had previously, with 

 Mr. Whitney, tract d these shales ai'ound the north side of Lake Huron and 

 Lake Michigan to Pointe aux Bales, and thence along Green Bay to Lake 

 Winnebago. These shales are however partly replaced by limestone in 

 Ohio, <fec. 



