wherry: clay from volcanic dust 577 



50 miles further northwest, a thick bed of similar clay (there 

 called bentonite) occurs at approximately the same geological 

 horizon, but the details of the section are so different that the 

 equivalence of the two deposits can not be regarded as estab- 

 lished. These stratigraphic relations are shown in the columnar 

 sections in figure 1. 



The continuity of practically every bed of clay and the com- 

 paratively slight variation in thickness shown by them through- 

 out the entire distance represented, over 50 miles, are well 

 brought out by these sections. The most remarkable instance 

 of this continuity is in one of the f-inch clay beds that lies about 

 5 feet above the main (3 to 4-foot) bed; it can be recognized in 

 every exposure. 



Character. The clay forms dense, compact masses, breaking 

 with a conchoidal fracture. When fresh it is deep gray, but on 

 weathering it becomes yellowish, owing to oxidation of the iron 

 compounds present. The weathered outcrops are strewn with 

 innumerable gypsum crystals, resulting from the action of sul- 

 furic acid (arising from oxidation of pyrite, which is abundant in 

 the shale and in places occurs in the clay itself) on calcite, which 

 is distributed through both rocks. 



On treatment with water the clay swells up conspicuously, 

 absorbing somewhat more than its own weight of water and 

 becoming sticky or plastic; on adding an excess of water it 

 slumps down to a flocculent slime. This swelling is evidently 

 due to the absorption of water by porous, spongelike, clay 

 particles. 



Under the microscope the clay shows an extremely fine grain. 

 When immersed in oil the individual particles, in so far as they 

 can be made out, exhibit slight, variable, but almost universal 

 double refraction, brought out more especially by the insertion 

 of a sensitive-tint selenite plate. Immersion in water, on the 

 other hand, causes a marked diminution in the double refraction, 

 many grains becoming quite isotropic. The index of refraction 

 of the clay, determined by the immersion method, is about 1.50; 

 but it varies, of course, with the water content, and, because of 

 the porous character of the particles, tends to increase slightly 



