580 



wherry: clay from volcanic dust 



No quartz has thus far been detected in any sample from the 

 region. Some clumps of clay are not flocculated, but remain 

 behind with the sandy residue. These probably represent in- 

 completely disintegrated grains of the original material from the 



TABLE 1 

 Analysis of Clay 



1. Analysis of fresh clay from bottom of main bed, 8 miles south of Buffalo 

 Gap, South Dakota; partial analyses of other samples show variations of 1 or 2 

 per cent in the several constituents, but not enough to affect materially the con- 

 clusions here drawn. 



2. .Same, recalculated to 100 per cent after deducting H,0 and CO £ and the 

 amount of CaO corresponding to the latter. 



3. Composition of average mica-andesite, after Osann, in Daly, Igneous rocks 

 and their origin, Xew York, p. 26, 1914. 



decomposition of which the clay has been derived. While no 

 complete, conclusive proof as to the nature of this substance is 

 obtainable, the irregular shapes and absence of crystal outline 

 of these masses, together with their isotropic, amorphous char- 

 acter, lead to the inference that they originally consisted of 

 volcanic glass. This, with the minerals above noted, would 

 constitute a mica-andesite rock. 



The chemical composition of the clay is given in table 1. 



