16 CALIFORNIA ACADEMY OF SCIENCES. 



Diabases are also found near the granite contact just west 

 of Real del Castillo. Under the microscope these rocks 

 prove to have a composition identical with that of diabase, 

 while the structure varies considerably in different speci- 

 mens. All are more or less uralitized, a process which is 

 generally considered to indicate dynamo-metamorphic ac- 

 tion; this fact thus also tends to prove that the diabases 

 form a part of the highly compressed slate series. The 

 most plausible view is, perhaps, to regard them as heavy 

 effusive masses poured out on the bottom of the sea, in 

 which the sediments of the metamorphic slate series once 

 accumulated or as intrusives in the slates before their 

 compression. Neither the collection nor the observations 

 are extensive enough for a detailed examination as to the 

 genesis and relations to other parts of the slate series. 

 Some of these rocks are certainly eruptive diabases or dy- 

 namo-metamorphic forms of these, while it is not impossi- 

 ble that others may be products of recrystallization of clas- 

 tic rocks, induced by dynamic processes. 



A specimen from the head of Gallo Canon, near the gran- 

 ite contact is macroscopically a greenish - gray middle- 

 grained diabase; in thin section it is seen to have a 

 hyphidiomorphic diabasic-granular structure, consisting 

 of Jathlike, clear and striated plagioclase crystals, pro- 

 bably oligoclase or andesine and allotriomorphic augite, 

 slightly brownish, filling the interstices between the former. 

 The augite is extensively uralitized, while the resulting 

 uralite again is decomposing into aggregates of chlorite and 

 serpentine. 



In another specimen from the foothills of San Rafael 

 Valley, along the road from Ensenada to Real, the uraliliza- 

 tion has progressed still further; the lathlike feldspar crys- 

 tals are still visible, and between them lies a little quartz 

 in grains, the shape of which is determined by the former. 

 No augite remains; the irregular grains of a light green 

 uralitic hornblende (a, light yellowish; b, brownish-green; 



