334 



REPORT OF NATIONAL MUSEUM, 1899, 



Nos. T303r), GlO-tl, 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- 

 ])onate and alkalies and the extremely finely comminuted siliceous material to w hiclif 

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

 smooth and plastic Ijluish-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 ^ as having- the 

 following composition: 



Vitrified brick for street pavements are made from fusible claj^s, 

 itiometimes in their natural condition and sometimes mixtures of ground 

 shale and clay. (See Specimens, Nos. 61141, 611-42, and 68049, from 

 Evansville, Indiana.) 



The following analyses of the materials used b}^ the Onondaga Vit- 

 rified Pressed Brick Company show the character of the materials 

 there used:^ 



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

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



^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. 



