H. Ries — A Peculiar Type of Clay, 



317 



scope show a variable number of irregular grains, some of them 

 of siliceous character, as well as a large quantity of very small 

 particles, which may be bunched together, and are in part of 

 colloidal character, so here again the Texas clay shows an 

 exceptional texture. 



Finally in addition to the peculiarities already mentioned we 

 may consider the cause of its plasticity. 



Various theories have been advanced to explain the plastic- 

 ity of clay, and a most interesting summary of these has been 



Fig. 1. 



Fig. ]. Photomicrograph of dolomitic clay, showing the rhomb-shaped 

 grains composing it. x 160. 



given by N. B. Davis. ^ As he points out in the conclusion of 

 his paper, plasticity seems to be due to the existence of gelati- 

 nous or colloidal matter, which may be of organic or inorganic 

 nature, but that tlie effect of these materials may be further 

 modified by absorbed salts, the relative proportions of large and 

 small grains, and perhaps by the size of the grains themselves. 

 This view no doubt applies to most clays, but the question 

 may be raised as to whether it fits the present case. If so, then 

 it must require an exceedingly small amount of colloidal mate- 

 * Amer. Inst. Min. Engrs., Trans., vol. li, p. 451, 1916. 



