CHEMUNG PERIOD. 287 



ing conditions and also for changes in the living species by exter- 

 mination. 



Moreover, while these mud-accumulations were here in progress, 

 there were Hamilton limestones forming in some of the Western 

 States, indicating again the existence of the interior or Mississippi 

 sea, — a feature in a large part of both Silurian and Devonian 

 geography. 



The Appalachian region is still an area of vastly the thickest 

 deposits, and hence of the greatest change of level by subsidence ; 

 and the great thickness of the formation (1000 feet) in central New 

 York makes it another example of the prolongation of the subsid- 

 ing Appalachian region northward over southern New York. This 

 fact and the thinning of the beds towards the Hudson indicate 

 that the Green Mountain region was at least a few feet above the 

 sea, so that the great New York bay, alluded to in the observa- 

 tions on the Oriskany beds, was still outlined on the east, although 

 communicating westward more or less perfectly with the interior 

 basin. 



Life. — The land-plants of the Hamilton beds prove that the 

 rocks and soil of the emerging continent and its islands were not 

 then barren wastes, whatever their earlier condition. There were 

 forests of Conifers and Lepidodendra, besides smaller plants, but no 

 Palms or Angiosperms (p. 165). 



CHEMUNG PERIOD (11). 



Epochs. — 1. Portage, or that of the Portage group (11 a) ; 

 2. Chemung, or that of the Chemung group (11 b). 



I. Rocks: kinds and distribution. 



Portage epoch. — The Portage group consists of shales and lami- 

 nated or shaly sandstones. In western New York, on the Genesee 

 River, the lower beds are the Cashaqua shales ; next, the Gardeau 

 shales and flagstones ; then, above these, the thick Portage sand- 

 stones. But going westward the shales increase in proportion, 

 and eastward the sandstones greatly predominate, the subdivisions 

 not continuing distinct ; there are also changes in the fossils cor- 

 responding with these variations. 



The rocks have a thickness of 1000 feet on the Genesee River, 

 and 1400 near Lake Erie. (Hall.) They are well developed about 

 Cayuga Lake, but have not been recognized in the eastern half of 

 the State of New York, v 



