10 STRATIGKAPHICAL GEOLOGY. 



loses the fossils which were typical of it, and becomes charged with fossils pecu- 

 liar to calcareous strata. Thus, while we find its lower beds, from Wayne county 

 Westward to the Niagara river, characterized by peculiar fossils, we find the 

 upper beds containing many species which pass upward into the Niagara Group. 

 Indeed, there is no line which^anbe designated between these two Groups, which 

 shall mark the limits of the organic products. It is true, nevertheless, that by 

 far the greater part of the fossils of the two Groups are distinct; and the small 

 number in the lower Group, of those which we regard as proper to the Niagara 

 Group, are for the most part inconspicuous, and not so well developed as they are 

 in the Niagara." Again, he says: 



" In tracing the Clinton Group Westerly, we find its affinities more with the 

 rocks below, or that the material and fossils recognized on the one side as the 

 Clinton formation are not strongly separated from the upper beds of the Hudson 

 River Group; and studied in these localities alone, they might be regarded as 

 constituting part of the same. On the other hand, the Niagara becomes defined 

 as a calcareous Group, and the line between it and the strata below is strongly 

 drawn. The base of this limestone would everj'^where be recognized as the base 

 of the Upper Silurian Rocks, while the strata below are marked by fossils which 

 belong to the Lower Silurian fauna." 



The Niagara Group consists of shales and limestones, and may, for the pur- 

 poses of this introduction, include the lenticular mass of dolomitic limestones 

 found in Canada, and bearing the name Guelph Group. The Guelph Group takes 

 its name from the town of Guelph in Canada, where it is about 160 feet in thick- 

 ness. At Lockport and at Niagara Falls, the Niagara Group consists of about 

 80 feet of shales, and 164 feet of limestones. The Group is found exposed in 

 Ohio, Indiana, Illinois and other Western States, rarely exceeding 400 feet in 

 thickness; but in Tennessee it reaches 1,700 feet, and is subdivided as follows: 

 1st, Clinch Mountain sandstone, consisting of shales and sandstones, 700 feet; 

 2d, White Oak Mountain sandstone, 500 feet ; 3d, Dyestone Group of shales and 

 sandstones, which takes its name from an iron ore, which is sometimes used as a 

 dyestone, 300 feet; aud 4th, Meniscus Limestone, which takes its name from a 

 lens or a meniscus-shaped fossil sponge, named by Roemer, Astrceospongia 

 meniscus, 200 feet. 



Prof. Hall says : " The rocks of this Group, where best developed in Western 

 New York, consist of a mass of shale, succeeded by one of limestone, the passage 

 from the former to the latter taking place by the gradual increase of calcareous 

 matter. The upper or terminating limestone of the Clinton Group is succeeded 

 by a soft argillo-calcareous shale, which maintains its character unchanged for a 

 thickness of 80 to 100 feet. Throughout the greater part of this it abounds in 

 fossils, nearly all of which are distinct from those in the beds of the Clinton 

 Group. In the Western part of New York, the lithological characters of the 

 Clinton and Niagara Groups are so similar, that they could well be united. The 

 fossils also of the two Groups, though generally distinct, are nevertheless generic- 

 ally similar, and several species pass from the lower to the higher Group. Still 

 farther West, the assimilation becomes more perfect, and there appears to be 

 no line of separation between the two Groups. At the same time the fossils 

 appear to be commingled." 



