rouruvRiES. 145 



Had tlie porphyries found in the Ohico conglomerate been ejected after 

 the Post-Neocomiau upheaval they would almost certainly have been de- 

 tected in the extensive exposures examined at and to the south of New 

 Idria. The porphyry at this point is therefore probably of nearly the same 

 age as the similar rock found associated in the Knoxville conglomerates 

 with granite pebbles and with fossils characteristic of the Knoxville series. 

 The eruption of this porphyry must antedate a part of the Knoxville period, 

 and the negative evidence is that it preceded the entire group of strata in 

 which the fragments occur. Like the porphyry found in the granite at 

 Steamboat Springs it is not improbably little younger than the granite. 



Diabase from Steamboat Springs. — Auioug tlio greatly disturbcd, partially meta- 

 morphosed, highly inclined, sedimentary rocks of Steamboat Springs, which 

 are overlain by andesites and basalts, occur some conglomerates. In these 

 were found dark pebbles of a crystalline rock sti'ikingly resembling the 

 material which forms the east wall of the Comstock lode in Vii'ginia City. 

 Under the microscope these pebbles proved to be plagioclase-pyroxene 

 porphyries with a crystalline groundmass. The pyroxenes are entirely 

 decomposed, but the chloi'ite and other decomposition products retain the 

 characteristic forms of augite or hypersthene. To which of these minerals 

 the original substance belonged cannot therefore be told. This rock is 

 microscopically as well as macroscopically indistinguishable from the por- 

 phyritic diabase of the Comstock. The beds in which the pebbles occur 

 are at least as old as the Mesozoic.^ 



LAVAS. 



Volcanic rocks of Steamboat Springs. — The audcsitcs aud basalts of Stcauiboat 

 Springs form a most interesting series both in themselves and because they 

 throw some additional light upon the important occurrences near the Com- 

 stock lode, which is only six miles from the Springs in a straight line and 



• I Lavo already drawn atteatiou to the rocks of Steamboat Springs in a paper entitled " Wasboe 

 rocks" : Bull. California Acad. Sci. No. G, 1836, p. 9:3; see, also, my paper On the texture of nias.sive 

 rocks: Am. Jour. Sci., :!d series, vol. 33, 1887, p. 50; and Bull. U. S. Geol. Survey No. 17, On the Develop- 

 ment of Crystallizatioa in tbo Igneous Rocks of Wasboe, Nevada, witb Notes on the Geology of tbo 

 District, by Messrs. Hague and Iddings. 



MON XIII 10 



