316 J. D. Dana on American Geological History. 



crease in the depth, of the continental seas, — an instance of the 

 oscillation of level to which the earth's crust was almost unceas- 

 ingly subject through all geological ages until the present. 



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

 tinent, 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 Hudson Period ; and with it, the Lower 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 

 material evidence of an epoch of violence and catastrophe : and 

 with this deposit the Upper Silurian began. 



The Upper Silurian has also its three great periods, — the 

 Niagara, the Onondaga, and the Lower Helderberg, besides 

 many subordinate epochs, — each characterized by its peculiar 

 organic remains,- — each evidence of the nearly or quite universal 

 devastation that preceded it, and of the act of omnipotence that 

 reinstated life on the globe, — each, too, bearing evidence of shal- 

 low or only moderately deep waters when they were formed ; and 

 the Onondaga Period — the period of the New York salt rocks — 

 telling of a half-emerged continent of considerable extent. 



Another devastation took place, and then opened, as De Yer- 

 neuil has shown, the Devonian Age or Age of Fishes. It com- 

 menced, like the Upper Silurian, with coarse sandstones, evidence 

 of a time of violence ; these were followed by another grit rock, 

 whose few organic remains show that life had already reappeared. 

 Then another change, — a change evidently in depth of water, — 

 and limestones were forming over the continent, from the Hud- 

 son far westward : the whole surface became an exuberant coral 

 reef, far exceeding in extent, if not in brilliancy, any modern 

 coral sea ; for such was a portion, at least, of the Upper Hel- 

 derberg Period. 



Again there was a general devastation, leaving not a trace of 

 the former life in the wide seas ; and where were coral reefs, es- 

 pecially in the more eastern portion of the continental seas, 

 sandstones and shales accumulated for thousands of feet in thick- 

 ness, with rarely a thin layer of limestone. Thus passed the 

 Hamilton, Chemung and Catskill Periods, of the Devonian 

 age. The life of these regions, which in some epochs was ex- 



* 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 Niagara limestone in Iowa and Wiscon- 

 sin by thick strata of Hudson River shales, giving a prolongation to these shales 

 before unsuspected. He had previously, with Mr. Whitney, traced these shales 

 around the north side of Lake Huron and. Lake Michigan to Pointe aux Baies, and 

 thence along Green Bay to Lake Winnebago. These shales are however partly re- 

 placed by limestone in Ohio, etc. 



