FIFTH REPORT OF THE DIRECTOR I908 



169 



portance for the understanding of the geological structure of the 

 eastern United States. 



As a third group of depositional inliers may be distinguished the 

 relatively rare and unimportant case where current mounds and 

 ridges have been buried under younger sediments and later become 

 exposed as inliers. An example of this kind is furnished by the 

 Le Claire limestone of Iowa 1 that forms mounds 50 feet high or 

 over and in regard to which it was already suggested by Hall that 

 " at the close of the Niagara huge mounds and ridges were built 

 011 the bottom of the shallow Silurian sea, in part by the accumu- 

 lation in situ of corals, crinoids and molluscous shells, and in part 

 by the drift of calcareous sediments under strong currents." 



Fig. 6 Lens or reef resting upon Clinton limestone and extending into Rochester shale. 

 (Copy from Sarle) 

 Fig. 7 Same worn down and forming inlier 



As these current mounds later project from the softer and more 

 easily eroded rocks that once buried them, and thus form inliers, 

 so also limestone mounds which grow on the bottom of the ocean 

 through the action of corals, bryozoans and the accumulation of 

 mollusks, may become buried in shales and later project from these, 

 as " lenses." Such reef structures have been described by Sarle 2 

 as reefs in the Clinton formation of western New York. When 

 they indicate an older horizon than the overlying shale, they con- 

 stitute true inliers. This is frequently the case in western New 

 York, where they represent the top of the Clinton limestone and 



1 Iowa Geol. Sur. Rep't, 11 :30s. 



2 Sarle, C. J. Amcr. Geol. 1901. 28 :282. 



6 



■ 



