334 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



of that of California, though no known fact lends this supposition proba- 

 bility. A prolonged, but wholly unsuccessful search was made for fossils, 

 which are also very rare in the other Mesozoic rocks of Nevada. 



A portion of these rocks are conglomerates. Among the pebbles of 

 these conglomerates are some of diabase, identical with the porphyritic 

 diabase which forms the hanging wall of the Comstock lode. There can be 

 little doubt that these pebbles formed a portion of masses which were erupted 

 simultaneously with that at Virginia, and it is possible that they came from 

 that very mass, though this is not certain. I did not succeed in finding peb- 

 bles of the still older diorite of Mt. Davidson, which may perhaps have been 

 bui'ied under the diabase at the time when the conglomerates were laid 

 down. The bearings of the occurrence of diabase pebbles at Steamboat 

 on the geology of Washoe I have enlarged upon in another publication.^ 



Earlier hornblende-andesitc. TllC oldcSt of tllO laVaS of the district is llOm- 



blende-andesite. It overlies the metamorphic beds in part and is distinctly 

 more recent than the date of their upheaval. All this lava is very compact and 

 of a bluish tint, but it is divisible into three varieties: one extremely fine- 

 grained and often of a slaty texture ; one a comparatively coarse-grained, 

 porphyritic rock ; the last glassy. Of these the first is most common. It is 

 also found near the Ophir grade at Virginia* and the two occurrences are 

 indistinguishably similar. The coarse-grained, somewhat grayish porphyry 

 is best represented at Steamboat, near the eastern edge of the map. At 

 Virginia this modification is perhaps the commonest. The glassy variety, 

 associated with the others, is found at the western edge of the map of Steam- 

 boat Springs. In this area there also occurs a very small amount of a 

 pyroxenic andesite which, after careful study, seems to me to represent a 

 strictly local variation in mineralogical composition and to pass over into 

 the hornblende rock by transitions. The fact that this andesite overlies the 

 metamorphics seems to indicate that it is later than the early Cretaceous. 

 The existence of glassy modifications forming a portion of the areas is evi- 

 dence that it is much later than the Cretaceous. Indeed it is hard to under- 

 stand how the glass can have failed to be removed if this andesite is older 

 than the Pliocene. 



I The Washoe rocks : Bull. California Acad. Sci. No. fi, vol.'.', 1637, p. 93. 



