74 



University of California. 



[Vol. 2. 



from the center and edges showed an interesting association of min- 

 erals. The white mineral from the center resembles feldspar, but 

 shows no twinning, and is intergrown in a micropegmatitic manner 

 with quartz. The angle of extinction of the supposed feldspar 

 measured on a cleavage crack is very low. The section prepared 

 from the edge showed only a small amount of quartz intergrown as 

 described, but numerous lath-shaped feldspars twinned according to 

 the albite law. Owing to alteration the extinction angles were diffi- 

 cult to measure, but those obtained indicated that it belonged 

 between albite and labradorite. In portions of the slide the feldspar 

 was observed undergoing alteration to the same cloudy isotropic 

 substance already described. The slender crystals observed in the 

 hand specimen proved to be a pale, slightly pleochroic hornblende. 

 This was shown by the extinction angle of twinned individuals, 

 which is 1 8°, and by characteristically shaped cross sections with 

 the usual cleavage. The pale color of the hornblende, taken 

 together with its specific gravity, which was found to be 3.065, indi- 

 cates that it is a variety intermediate between tremolite and ordinary 

 hornblende, a hornblende poor in iron. In other sections of similar 

 occurrence many of the individuals appeared perfectly colorless, and 

 are undoubtedly closely allied to tremolite. The specific gravity of 

 the granophyric intergrowth was found to be 2.654. 



In the case of another dike, a layer of brown hornblende forms 

 the edge, lying between the feldspar and the serpentine, though 

 sometimes separated from the latter by a thin sheet of feldspar. 



Turner* has described veins in serpentine from Meadow Valley 

 in the Sierra Nevada. The feldspar was proved to be albite by 

 analysis. Similar veins have also been seen by the author in the 

 serpentines in other places in the Coast Ranges. 



Age. — The field relations give no clue to the exact age of these 

 rocks. They are older than the Miocene, but beyond that we can 

 go only by their logy with similar eruptives in this part of the 

 Coast Ranges. In the San Rafael Mountains, 40 miles to the south- 

 east, there is a large body of serpentine having associated with it 

 dikes of diabase. The whole has been intruded into rocks of sup- 



* American Geologist, Vol. XIII, p. 303. 



