472 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 5° 



and on the bottom of which accumulated, during periods of drought, 

 the deposits of carbonate of lime and gypsum so characteristic of 

 the playa lakes of the West. This naturally thins out along the 

 margin where it overlaps the fragmental material from the steep 

 slopes. None of this needs attention in the discussion of the present 

 problems. The crushed and metamorphosed white sandstone under- 



Fig. 126. — Showing the Microstructure of the Rock Flour. The angular 

 particles are all of quartz. 



lying it needs, however, careful consideration. This has been already 

 the subject of brief notice by the writer, 1 but is of sufficient im- 

 portance, as bearing upon the matter of origin of the crater, to be 

 elaborated here. 



Petro graphic Description of Rock Products. — The unaltered gray 

 sandstone, which has already been referred to as underlying the 

 limestone and having an approximate thickness of 400 or 500 

 feet, is in its typical form of a very light gray or nearly white 

 color, and is composed wholly of well-rounded, clear, colorless 

 grains of quartz sand, with an occasional fragment of feldspar. 

 A photomicrograph of this is given in figure 1, plate lxxi. This 

 rock, as shown in the borings and as noted in the description of the 

 crater walls, is often much shattered and crushed and is found in 



'On a Peculiar Form of Metamorphism in Siliceous Sandstone, Proc. U. S. 

 Nat. Mus., vol. xxxii, 1907, pp. 547-550. 



