GEOGRAPHIC FEATURES 229 



The rhyolite tuffs often weather in curious forms. Figure 1, plate 6, 

 represents a group of little white cones at the northwest base of the 

 Palmetto mountains, in the little cavities of which small rodents make 

 their nests; figure 2, plate 6, shows a single one of these cones, which is 

 perhaps 20 feet in height. 



DESERT VARNISH 



On the detrital slopes many of the boulders of lava and other rocks 

 are covered with a dark brown shining coating, known as desert varnish. 

 Some of this material was examined by Dr H. "N. Stokes, who found it 

 to be chiefly manganese dioxide. Under this outer coating in the basalt 

 boulder examined, there is a decomposed layer in which Doctor Stokes 

 determined the presence of a silicate soluble in hydrochloric acid and 

 containing alumina and lime. This silicate is not feldspar, since it is 

 soluble in acid. Oscar Loew 5 regards the desert varnish of the Mojave 

 desert as having been deposited on the rocks as carbonate of manganese 

 by the retreating waters of a shallow ocean which he supposes once 

 covered that desert. The binoxide of manganese was then formed from 

 the carbonate by the action of sunlight and air. Doctor Loew placed a 

 granite boulder weighing 80 grams in hydrochloric acid until it showed 

 its natural color. The material in solution in the acid was as follows : 



Grams 



Sesquioxide of iron . 078 



Binoxide of manganese 0.038 



Oxide of nickel trace 



Dr G. P. Merrill 6 investigated the desert varnish on pebbles of quartz- 

 ite in Toole valley, Utah, which was formerly covered with the waters 

 of lake Bonneville. He found that the coating gave reactions for iron 

 and manganese. He writes: 



"It is evident that the exterior coloring of the desert varnish is due mainly 

 to a local segregation of oxide of iron with a little manganese and organic 

 matter. All things considered, it seems safe to assume that, this local discolor- 

 ation is due to a superficial segregation of the metallic contents of the quartz- 

 ite in a state of higher oxidation, the iron originally in the form of a carbonate 

 being converted into a hydra ted oxide, while the lime carbonate itself was re- 

 moved in solution. The small amount of organic matter may have been added 

 from external sources from the water of the original lake." 



There is no evidence that the detrital slope in Clayton valley from 

 which the specimen of desert varnish examined by Doctor Stokes was 



5 Wheeler Survey ; Annual Report for 1876, Appendix JJ, p. 179. 

 8 Bulletin no. 150, TJ. S. Geological Survey, pp. 389-391. 



