Page:The Journal of geology (IA journalofgeology11893univers).pdf/156

bearing upon the phenomena here discussed is likely to be found in that region.

In southern Oregon, on the line of the Southern Pacific Railway, the writer has on several occasions observed the eruptive contact of an extensive granite mass against sedimentary strata which have been mapped as "Auriferous slates," which are probably early Mesozoic or Carboniferous in age. The intrusion of the granite into the sedimentary rocks is unquestionable, the relations being well exhibited in the excellent exposures afforded by the railway cuttings.

In California the statements of Whitney^[1] and the more recent writings of the geologists of the U. S. Geological Survey, Diller, Becker, Turner, and Lindgren, and of Mr. H. W. Fairbanks, seem to leave no room for doubt that a great part, probably the greater part, of the granitoid rocks of the Sierra Nevada is of Mesozoic age, and has invaded the now more or less altered sedimentary and volcanic rocks known as the "Auriferous slates," which range in age from the Silurian up to the Jurassic.

Here again we have clearly to deal with a granitic batholite which must, by absorption or otherwise, have replaced a large portion of the preëxisting lower rocks in the region affected. From the facts recorded by able and critical observers, this conclusion holds, notwithstanding the probability that there may also be remnants of an older granite to be discriminated from the Mesozoic mass. In the southern Sierra, as Becker has, with wise caution, pointed out, we approach the region of Archæan granite known in the Grand Cañon section. It would therefore be not at all remarkable to find these more ancient granites involved with the newer in the Sierra Nevada. But their presence could not affect the important fact of an invasion of the crust during middle Mesozoic time by an immense granitic batholite, which invasion without doubt had much to do with the metamorphism of the strata which survived the upward progress of the magma into the crust.

Here again the development of the batholite seems to have