124 NEW YORK STATE MUSEUM 



the Niagara period, when the Coralline limestone of Schoharie was 

 laid down. Westward, however, the adjustment of conditions went 

 on more rapidly, and the Clinton limestones, with the calcareous 

 shales and limestones of the upper Niagaran, became the charac- 

 teristic deposits. During nearly the entire Niagara period life was- 

 abundant in the Siluric sea, and the Bay of New York had its mar- 

 velous succession of faunas, which have made these strata the stand- 

 ard for the Siluric beds of this continent. 



All the Siluric limestones of the Niagara section show characters 

 pointing to a fragmental origin, and in this respect they contrast 

 strongly with the Devonic limestones in the southern part of the 

 district. The latter, as before mentioned, show the characteristics 

 of an ancient coral reef, and we may therefore assume that they 

 were built up in situ by the polyps and other lime-secreting or- 

 ganisms. Not so with the Siluric limestones. These, to be sure, 

 were derived from similar deposits by lime-secreting organisms, but 

 these deposits were originally made in a different place from that 

 in which we find the limestones today. A sedimentary limestone or 

 lime-sandstone is similar to a quartz sandstone or a shale, in that 

 the material of which it is formed is the product of erosion of pre- 

 existing rocks. In the case of the quartz sandstone, this is gen- 

 erally an inorganically formed rock, while the sedimentary lime- 

 stones are most usually derived from organically formed rocks. In 

 the former case, the source of the material is often a distant one,, 

 while in the latter it is generally, though not necessarily always, 

 close at hand. A coral reef growing in moderately shallow water 

 is attacked by the waves, as are all rocks which come within their 

 reach. Erosion results, and the product of this activity is carried 

 away and deposited on the ocean floor as a calcareous sand. Thus 

 stratified deposits of limestones are formed, whereas in the original 

 organic reef, no stratification is to be expected. In the immediate 

 neighborhood of the growing reef, the beds of calcareous sand will 

 slowly envelop the original deposit from which they were derived,, 

 and thus the source of supply is chiefly the upper growing portion 

 of the reef. On the lime-sandstone strata which flank the reef, in- 

 dependent masses of coral may at times grow, while other or- 



