8 CALIFORNIA ACADEMY OF SCIENCES. 



clastic feldspar prevails — as would be the case along the 

 profile from Baja California, described in these notes — 

 would then be indicated as a dioritic facies of Gfranite. 



o 



We have, however, as yet, very few reliable petrographical 

 determinations in this area, but the few available seem rather 

 to indicate that a large portion of it consists of rocks in 

 which plagioclase prevails."^ Future examinations will show 

 the extent of these dioritic areas and their connection with 

 the granite, 



II. — PaL.EO VOLCANIC EFFUSIVE KOCKS. 



Quavtz-porphyrites. 



Heavy masses of effusive rocks cover the granite almost 

 everywhere in the country adjoining Todos Santos Bay. 

 Sometimes these eruptives form massive, high ranges, like 

 that extending from Punta Banda to Santo Tomas, deeply cut 

 into by erosion; or, again, like the hills near San Carlos and 

 north of Ensenada, perfectly distinct flows or sheets may be 

 observed, nearly horizontal at the coast but inclining at a 

 steeper angle further inland. Considerably altered tuface- 

 ous masses sometimes appear in connection with them. 

 Nearly all of these rocks have a porphyritic habit and are 

 very acid, while the feldspars are predominantly plagio- 

 clastic. On the whole, they maybe characterized as quartz- 

 porphyrites and quartz-hornblende porphyrites, although 

 the latter are comparatively less extensively developed. 

 Fragments and boulders of these porphyrites are included 



*Ad. Schmidt (L.J., 1878 716) examined a specimeu from the YosemiteValley.and found 

 it to be a quartz diorite. He also pointed out the great general similarity of this quartz 

 diorite (identical with the quartz-mica diorite described in this paper) with the Toualite 

 (V. Rath), occurring as an intrusive massive in Southern Tyrol at Mt. Adamello. 



An analysis of the Yosemite granite in King's Fortieth Parallel (Systematic Geology, p. 

 Ill), shows 4.49 per cent. Na,p, and only 2.08 per cent. Ka 0, thus confirming Schmidt's 

 results. 



A granite from Steamboat Springs, Nevada, near the east base of the Sierra Nevada, 

 carries apparently about equal quantities of orthoclase and plagioclase. (G. F. Becker, 

 Quicksilver Deposits of the Pacific Coast, Monograph XIII, U, S. Geol. Survey. 



