334 



REPORT OF NATIONAL MUSEUM, 1899. 



Nos. 73036, 61041, and 61042, of these clays in their natural, mixed, 

 and baked condition.) 



One of the most constant distinctions between the so-called clays of glacial and 

 nonglacial origin, are the relatively large amounts, in the first mentioned, of lime car- 

 bonate and alkalies and the extremely finely comminuted siliceous material to which 

 the name rock flour is commonly given. Prof. W. O. Crosby, has shown that the 

 smooth and plastic bluish-gray brick clays of West Cambridge contain only from 

 one-fourth to one-third their bulk of the clay kaolin, the remainder being largely 

 rock flour. [Proceedings of the Boston Society of Natural History, XXV, 1890.] 



Leda clays from Beaver County, Pennsylvania, used in the manu- 

 facture of terra cotta at New Brighton, are reported x as having the 

 following composition: 



Vitrified brick for street pavements are made from fusible clays, 

 sometimes in their natural condition and sometimes mixtures of ground 

 shale and clay. (See Specimens, Nos. 61141, 61142, and 68049, from 

 Evansville, Indiana.) 



The following analyses of the materials used by the Onondaga Vit- 

 rified Pressed Brick Comjj *\y show the character of the materials 

 there used: 2 



The name slip clay is given to a readily fusible, impalpably fine clay 

 used for imparting a glaze to earthenware vessels. These clays carry 



1 Second Geological Survey of Pennsylvania, Report of Chemical Analyses, p. 257. 



2 Bulletin of the New York State Museum, III, No. 12, March, 1895. Clay Indus- 

 tries of New York, p. 200. 



