GEOLOGY. 257 



possible to avoid the conclusion that this lower limestone of Tennessee 

 was the equivalent of the Black River limestone of New York, and that 

 perhaps, also, a portion of the birds-eye limestone was included with it. 

 In the second and third divisions there were about forty species of 

 fossils common to those rocks, and to the Trenton limestone of New 

 York, making the conclusion unavoidable that the strata of these two 

 portions of country were of the same age. In the same manner it was 

 shown, that the Nashville group of limestones were probably identical 

 with the Hudson River group. In this connection, Prof. Hall showed 

 that the lower Silurian limestones, or those known as the Chazy, Black 

 River and Trenton limestones, thinned out in a north-westerly direc- 

 tion, till they were less than 100 feet thick on the north shore of Lake 

 Michigan, and the west shore of Green Bay ; that in Wisconsin they 

 were less than seventy-five feet thick, and at the Falls of St. Anthony, 

 on the Mississippi River, they were less than fifty feet thick. In Ten- 

 nessee, the same strata, as far as seen, were 200 feet thick, and the 

 base had not been seen. In like manner, the Hudson River group 

 gradually thins out in the west and north-west, its sandy portions dis- 

 appearing in Canada West ; and though eight hundred feet thick in 

 New York, is less than two hundred on the north shore of Lake Michi- 

 gan, and disappears entirely in Wisconsin. In Tennessee, the rocks 

 corresponding in position are nearly all limestones, and have a thick- 

 ness of about three hundred feet. The strata succeeding these lower 

 Silurian limestones are known as the gray limestone. From this lime- 

 stone, forty-two species of fossils had been collected, of which twenty- 

 seven were known species, and common to the rocks of New York, as 

 well as Tennessee. But what was very remarkable was the fact that 

 of these twenty-seven species, several were of the Niagara group, or 

 those known only in the rocks of that period, while others were known 

 only in the lower Helderberg limestones ; and others still were found 

 only in the upper Helderberg limestones, or the Onondaga and cor- 

 niferous limestones. Thus showing that rocks of the middle and 

 upper Silurian periods, and of the Devonian period, were here united 

 in one ; or that the formation was altogether so uniform and homo- 

 geneous, that no subdivision could be made. That the Onondaga 

 salt group of New York, having a thickness of 800 or 1,000 feet, has 

 entirely disappeared, and that the Niagara and lower Helderberg 

 limestones thus come in contact ; and, again, from the absence of 

 Oriskany sandstone, and Candigalli grit, the lower and upper Helder- 

 berg limestones are united ; and thus the three limestones, so widely 

 separated in New York, become, physically, one limestone in Tennessee. 

 Prof. H. showed that each of these periods in New York were marked 

 by the presence of more than three hundred species of fossils, and that 

 very few of these passed from one group to the next, showing, conclu- 

 sively, that they were distinct formations, and of distinct and succeed- 

 ing creations. He remarked that this collection of fossils afforded some 

 evidence of the influence of latitude upon the development of animal 

 life, and that climatic influences had prevailed at that early period as 

 well as in subsequent ones. Of the lower Silurian species, one half 

 were new, or unknown in the rocks of New York, of the same age, from 



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