THE WASHOE ROCKS. Ill 



able intermediate stage, from a mass like hard, white chalk 

 to a fresh andesite, is plainly visible on an unbroken expo- 

 sure. There are other exposures in abundance on the sur- 

 face. The analogy between this white rock and the felsitic 

 quartz porphyry depends on a single specimen of the former, 

 which shows a banded structure something like that of rhyo- 

 lite, a feature which is also of common occurrence in the 

 felsitic rock. Now, I have called attention to this struct- 

 ure of the east country rock in the following terms i^*^ 



*' In several of the rocks a stratified or laminated structure 

 is visible; but in the half-dozen such cases known to me, 

 the phenomenon extends for very short distances, often only 

 a few feet, and appears to be the result of some local varia- 

 tion in the composition of the rock; for not only can I 

 perceive no general uniformity in the direction of the layers 

 in these difi'erent spots, but I have a single hand specimen 

 which shows two sets of them at an angle of nearly 90^ to 

 one another." 



"There are limited occurrences of excessively fine-grained, 

 closely laminated diabase, resembling slate. The diorites 

 and both the andesites show the same phenomenon." 



The specimen of white rock supposed to be so significant 

 came from one of these spots, which occur not only in it but 

 in other rocks as well. The lamination, however, is not 

 characteristic but extremely exceptional in the white rock. 

 The specimen is not representative, but was carefully pre- 

 served as an exception, and the peculiarity which it presents 

 has no taxonomic value. 



Quartz porpJiynj. — Messrs. Hague and Iddings employ this 

 as it appears to me, wholly baseless identification, to argue 

 that the white rock containing no quartz excepting as a re- 

 sult of decomposition, is a dike of rliyolite, and proves that 

 my identification of tlie only quartzose rock in the district as 

 pre-Tertiary quartz porphyry is erroneous, as well as my in- 

 terpretation of its structural relations. A very large body 



Note '^. — Geology of the Comstock Lode, pages 51 and 182. 



