OBSERVATIONS BY PREVIOUS INVESTIGATORS. 497 



on granite, which forms the floor of the depression in which the town 

 of Ensenada is situated. These are consequently younger than the 

 granite, and their rolled fragments contained in the Cretaceous beds 

 prove them to be of pre-Cretaceous age. 



On the immediate coast north of Sausal are recent eruptions of basalt 

 of a somewhat unusual type which carries hypersthene and no olivine. 

 These recent flows extend about 20 miles northward along the coast. 

 The granite rocks forming the floor of the Ensenada basin are shown 

 by microscopic examination to be more properly quartz-mica-diorites. 

 About twelve miles east of Ensenada they are succeeded by a belt of 

 metamorphic slates, both chloritic and micaceous, standing nearly verti- 

 cal or with a steep dij^ to the east and associated with diabases. It is 

 these rocks that carr}^ the gold quartz veins, which are seldom, if ever, 

 found in the granites that surround and are in part covered by them. 

 They strike north-northeast and south-southeast. 



The bottom of the interior valley of San Rafael, fifteen miles long and 

 about ten miles wide, is 600 to 800 feet below the general level, a sunken 

 area between two north and south faults, as he assumes, with knolls of 

 slates and granite projecting through it. A short distance east of this 

 valley the granite hills merge into a gently sloping granite plateau, which 

 rises to an elevation of 5,000 feet and at one point is capped by an iso- 

 lated patch of auriferous gravels, compared by Lindgren to the auriferous 

 gravels of the Sierra Nevada. It then descends abruptly in a distance 

 of five miles to the level of the Colorado desert, no metamorphic rocks 

 being visible on the eastern escarpment at the point examined, but flat- 

 topped mesas of volcanic rocks being discernible at its base further south. 



The granite of the plateau, according to Lindgren, is a white coarse- 

 grained hornblende granitite, or granite rich in plagioclase, with a some- 

 what dioritic facies, as is the granite in the latitude of San Diego, and 

 resembles that of the Sierra Nevada. 



He considers that the present metamorphic areas are small remnants 

 of those that once covered the granite ; that the great eastern escarpment 

 is formed by an enormous fault of comparatively recent date, and that 

 the auriferous gravel near the summit of the plateau is part of an old 

 river channel that has been cut oft" by the fault. All these facts, together 

 with the long western slope of the range and the recent volcanic out- 

 bursts at the eastern foot, constitute a close parallelism in geological 

 structure between the peninsular Sierra and the Sierra Nevada. 



From Professor Gabb's description of the desert region between La Paz 

 and Santa Gertrudis (latitude 28° 30'), with regard to which his notes 

 are more complete than on the region further north, which he apparently 



LXIX— Bull. Geol. Soc Am., Vol. 5, 1893. 



