CLAYS OF NEW YOKK 573 



The mellowed outcrops of many of tlie sliale formations occur- 

 ring within the state should also, perhaps, be classed under the 

 head of residual clays. In the latter case however the clay is a 

 product of disintegration; in the former, of decomposition. 



2 Sedimentary clays. The soft plastic clays belong to three 

 geologic formations, Quaternary, Tertiary and Cretaceous. 



The first class is by far the most common. The second class 

 is somewhat indefinite in extent, but a large number of the Long 

 Island deposits probably belong to it.-^ Of the third ^lass theret 

 are undoubted representatives on Long Island and Staten Island,. 

 as well as some additional ones on Long Island, which are 

 questionable. The clays of the mainland are all Quaternary sO' 

 far as known. This does not include the shales which are treated 

 in a separate chapter. 



Many of the deposits are local and basin-shaped, lying in the 

 bottoms of valleys which are often broad and fertile. They vary 

 in depth from 4 to 20 or even 50 feet and as a rule they are 

 underlain by modified drift or by bed rock. The clay is gen- 

 erally of a blue color, the uppermost portion for a few feet being 

 weathered red or yellow. Stratification is sometimes present, and 

 streaks of marl are common. In some of the beds small pebbles, 

 usually of limestone, are found, and these have to be separated by 

 special machinery in the process of manufacture; at other localities 

 the clay is covered by a foot or more of peat. 



The basin-shaped deposits are no doubt the sites of former ponds 

 or lakes, formed commonly by the damming up of the valleys, and 

 filled later with the sediment of the streams from the retreating ice 

 sheet. The valleys in which these deposits lie are usually broad 

 and shallow, that in which the Genesee river flows from Mt Morris 

 to Rochester being a good example. The waters of the river were 

 backed up by the ice for a time, during which the valley was con- 

 verted into a shallow lake in which a large amount of aluminous 

 mud was deposited. This material has been employed for common 

 brick. 



1 F. J. H. Merrill. "Geology of Long Island," Ann. N. Y. acad- sci. Nov. 1884. 



