496 NEW YOKK STATE MUSEUM 



third class there 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. 

 The problems of the Quaternary formations in New York are by 

 no means solved, and it is not always possible to decide on the 

 causes leading to the deposition of any particular body of clay 

 by a single visit to the locality. 



A great majority 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 four to 20 or even 50 feet ; as a rule 

 they are underlain by modified drift or by bed rock. The clay 

 is generally of a blue color, the upper few feet being weathered 

 mostly to red or yellow. Stratification is rarely present, but 

 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. In many 

 instances 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 in many instances by the damming up of 

 valleys, which have been 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. The broad flat 

 valley in which the Genesee river flows from Mt. Morris to 

 Rochester is a good example. The waters of the river were 

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

 converted into a shallow lake in which a large amount of 

 aluminous mud was deposited. This material has been employed 

 for common brick. 



There are a number of these deposits which are of sufficient 

 interest, geologically as well as commercially, to be mentioned in 

 some detail. 



At Dunkirk is a bed of clay having a depth of over 20 

 feet. The upper six feet are yellow and of a sandy nature, while 

 the lower two-thirds is blue and of much better quality. It is 

 mentioned by Prof. Hall* in his report, and is an instructive 

 example of the manner in which the clay changes in color, down- 

 ward. 



* ieology of New York, 4th Dirtrict,.lc.4J, p. 332. 



