LOWER HELDERBERG PERIOD. 251 



shrinkage-cracks and other peculiarities that the layers were suc- 

 cessively formed in shallow waters, it follows that there must have 

 been a slow subsidence of the region during the progress of the 

 period, — it may have been but a few inches or feet in a century. 

 In water ten feet deep, a bed ten feet thick, or a little over ten, may 

 form ; but with a slow sinking, at a rate not beyond the rapidity of 

 deposition, the beds may reach any thickness, limited only by the 

 cessation of the subsidence. 



Life. — The half-submerged marshes or flats of this period, bordered 

 by dry land at least on the north, and probably including dry areas 

 over their surface, would have given an opportunity for forest-trees 

 to grow and forest-animals to live where their remains would be 

 sure to find a safe burial, as they did in after-times. Yet, as in the 

 Medina and Clinton epochs, no trace of a leaf, or stem, or relic of a 

 land or fresh-water animal has been afforded by the beds. 



The extermination of life at the close of the Niagara period was 

 very nearly complete in the region of New York, while apparently 

 only partial in the interior basin, as in Tennessee. The cause of the 

 extinction was connected, no doubt, with the changes that ushered 

 in the Salina period ; and the existence of seas over portions of the 

 interior accounts both for the limestones of the Salina epoch and 

 the continuation of a portion of the Niagara life beyond the termi- 

 nation of the period. The beds of the Mississippi basin require a 

 fuller elucidation for safe inferences. 



LOWER HELDERBERG PERIOD (7). 

 I. Rocks : kinds and distribution. 



The Lower Helderberg period was one of abundant marine life, 

 and of the formation of thick limestone strata, — a period, therefore, 

 not of prevailing salt marshes, but of clear seas over the submerged 

 land. 



The formation in New York has its greatest thickness at the 

 Helderberg Mountains south of Albany, where it is over 200 feet ; 

 and hence the name of the period. 



The beds extend westward in New York, and gradually thin out 

 in Ontario county, being wholly absent from the western part of 

 the State. The beds have been reported to occur in Ohio, Missouri, 

 and Tennessee ; but according to the more recent investigations 

 they are wanting over the interior basin. 



Near Montreal, in Canada, a small patch overlies unconformably 

 the Hudson River shales. 



