SEDIMKNTAUY UOCKS AT STEAMBOxVT. 333 



tially similar to that which appears near the southern end of the Conistock 

 Lode. The superficial exposure there is very small, but the rock extends 

 into the workings of the mines at American Flat. Similar o-ranite is found 

 near Washoe Lake, some miles south of Steamboat, and large exposures of 

 it exist between these points and Lake Tahoe, in the Sierra. 



The granite is intersected by distinct dikes of granite porphyry. So 

 far as observed these dikes do not penetrate the sedimentary rocks and are 

 ppobably older than the beds. 



The m=tamorphic series. — Upou tlio granitc Hos a couslderable area of sedi- 

 ments, which are in part very highly metamorphosed and in part but little 

 altered. These beds are usually nearly vertical and strike with the trend 

 of the Sierra. Attempts to make two series of them failed and only resulted 

 in showing that the metamorphism was partial and irregular. The highly 

 altered portions considerably resemble Archaean schists, but the partial 

 character of the metamorphism seems to forbid their reference to a Pre- 

 Cambrian age. They are certainly Pre-Tertiary, for there are Tertiary 

 strata within a few miles both to the north and to the south which are far 

 less disturbed and not at all metamorphosed. The only other sedimentary 

 rocks known to exist near the eastern edge of the Great Basin in this lati- 

 tude are those determined to be Jura-Trias by the paleontologists who dis- 

 cussed the fossils collected by the geologists of the Geological Exploration of 

 the Fortieth Parallel. Dr. White, on reviewing the evidence on which the 

 assignment was made, thinks it insufficient to justify any conclusion more 

 definite than that the beds in question are Mesozoic. The descriptions of 

 these Mesozoic rocks accord very well with the strata found at Steamboat. 

 Their metamorphism is perhaps an evidence that they are not younger than 

 the Neocomian, for no more recent alteration of any such intensity is known 

 to have taken place later than the Post-Neocomian upheaval so often re- 

 ferred to in this volume. The upheaval is the same as that called the Post- 

 Jurassic by Professor Whitney, and its existence was no doul^t given due 

 weight by the geologists of the fortieth parallel when they referred the 

 Mesozoic beds of Nevada to the Jura-Trias. As this region is separated 

 from the gold belt by a great range of mountains, however, it is not impo.s- 

 sible that its metamorphism may have been subsequent to and independent 



