20 GENERAL DISCUSSION OF CLAYS. 



nesia, may be present in some clays derived from mag- 

 nesian limestone, while the sulphate of magnesia or 

 ep^m salts when present, may aid in the formation 

 of a white coating on the surface of the ware; its pre- 

 sence can sometimes be detected by the bitter taste 

 which it imparts to the clay. 



The effects of magnesia in clays are considered to be 

 the same as those produced by lime. 



SILICA IN CLAYS. 



Three types of silica may be recognized in clay, i. e. 



1st. Quartz. 



2nd. That which is combined with alumina and 

 water in kaolinite. 



3rd. That which is combined with one or more 

 bases, forming silicate minerals, such as feldspar, 

 mica, etc. 



In chemical analysis the first and third are some- 

 times grouped together under the name of sand, jr at 

 times erroneously spoken of as free silica. 



The sand is practically insoluble in sulphuric acid 

 and caustic soda and this fact is utilized in the ration- 

 al analysis of clay. 



Few clays, so far as known, are free from quartz, 

 but it is present in variable amounts in different ones. 

 A minimum of .2 of one per cent, has been recorded 

 from New Jersey* while the average in the Wood- 

 bridge fire clays is five per cent. 



In the Missouri flint clays, a minimum of .5 of one 

 per cent., is recorded, while the sand percentage is 20 

 to 43 per cent, in the St. Louis fire clays, and 20 to 50 

 per cent, in the Loess clays. § 



27 samples of Alabama clays contained from 5 to 50 

 per cent, of insoluble residue. 



70 North Carolina clays had from 15.75 per cent, 

 to 70.43 per cent, of insoluble residue. 



In European clays similar variations are observ- 

 able. The most important effect of silica or sand is 



*G. H. Cook, Cl!ays of New Jersey, 1878, p. 273. 

 § Wheeler, Miasoupi Geological Survey, XI, page B4. 



