ANDESITES. 149 



wliere it should be exposed the ground is so covered with bowklers of tlie 

 pyroxene-andesite, which have rolled down from the adjoining liill, that 

 the croppings may have been concealed. Repeated and careful search was 

 made for them in vain. The transition andesite, though, like the others, 

 certainly younger than the earlier hornblende-andesite, does not stand in 

 structural relation to the other varieties. That it is very iiearlv of the 

 same age seems certain. In the Washoe district the micaceous rocks suc- 

 ceeded the pyroxene rocks of Mt. Kate,' but are connected with them by 

 transitions. On the other hand, Mr. Lindgren found that near Bodie pvrox- 

 ene outflows succeeded micaceous ones. 



It would be a very easy matter so to arrange the slides of the older 

 and younger andesites of Steamboat that they would seem to form a con- 

 tinuous series, and a tolerable argument might thus be offered for regarding 

 the rocks as substantially one. It would be more difficult to treat the hand 

 specimens in this way, while no observer could examine the rocks in place 

 at Steamboat without perceiving the difference in age and geological posi- 

 tion of the older and younger andesites 



The services which the microscope renders to lithological geology are 

 very great, but there are f;icts connected with this study which are not best 

 elucidated by examining the rocks a square millimeter at a time. It might, 

 indeed, be said that since these rocks are so closely allied it is useless to 

 make geological distinctions between them; but, if the purpose of lithology 

 is to ascertain the causes which underlie the variations in composition and 

 structure of rocks, the progresit' of this branch of science could only be 

 dela3ed by a failure to study and record all the distinctions which it is pos- 

 sible to trace. When these causes are once understood, and not before, it 

 will be possible to judge what differences are trul}' significant. 



A natural group of andesites. — A fact of souio Importancc, at Icast to the field 

 geologist, which is clearly brought out bv the study of the rocks of Steam- 

 boat Springs and confirmed b}' observations in California, is that there is a 

 group of comparatively recent andesites, varving considerably in mineral- 

 ogical composition, but possessing much macroscopical similarity and a 

 close geological relationsliij). Mo.st of the vounger andesites of Steamboat 



' Bull. Clifuniiii A'-aii. Sd. X(i. i\ vol. "J. IgSG, p. 09. 



