'318 R. Eies — A Peculiar Type of Clay. 



rial to develop the characteristic plasticity of clay, in this case 

 not more than \\ per cent, if we assume all of the iron oxide 

 and alumina to be in that form. 



While this small amount of gelatinous material may serve 

 as a contributory cause in this instance, it may well be ques- 

 tioned whether the shape of the dolomite rhombs does not 

 exert some influence, for it would seem that because of their 

 great number, and small size, the flat faces of many of these 

 must be in contact, or separated by a thin fllm of water when 

 the clay is wet. The surface tension would tend to hold the 

 surfaces of the grains together, but not prevent them slipping 

 under pressure, thus giving mobility to the mass, and increas- 

 ing its plasticity. 



The case is not entirely similar to the supposed effect of 

 plate structure, thought by some to be present in kaolin, and 

 to explain its plasticity, but it is more like that of flnely ground 

 calcite, which Wheeler states developed plasticity after treat- 

 ment in a ball uiill,^ with the difference that the small amount 

 of colloidal material may have helped to increase its plasticity 

 somewhat. 



Department Geology, 

 Cornell University. 



* Mo. Geol. Sarv., xi, p. 106, 1896. 



