35° 



RULIFF S. HOLWA Y 



order. While this mineral differs somewhat from the sphene appear- 

 ing in grains, it is thought to be the same, although possibly somewhat 

 changed by decomposition. 



Calaveras Valley. — This little valley is in Santa Clara county, 

 about fifteen miles northwest from the Lick Observatory. In it are 

 found two distinct kinds of eclogite — a light-green omphacite variety 

 and a dark, almost black, hornblende eclogite. The first occurs at 

 the mouth of the narrow gorge which is the outlet of the valley to the 

 north. The outcrop is on the west side of the stream on the bank 

 of the flood-plain, and is about a hundred feet long. On the hillside 

 above, serpentine is found, and in the stream-bed below, shale and 

 sandstone, with characteristic Miocene fossils. 



The dark hornblende eclogite is farther upstream on the east side 

 of the valley. The exposure is over a hundred feet in thickness, and 

 was followed southward for more than a half-mile without finding the 

 limit. On the upper side is a greenstone with an extensive serpentine 

 belt lying just above. Farther upstream sandstone occurs between 

 the eclogite and the serpentine. The eclogite appears to be a dike 

 cutting across the Franciscan sandstone. 



The green eclogite of the lower exposure varies considerably in its 

 appearance. The garnets are sometimes small and clear-cut, some- 

 times 8 to lo™'" in diameter and without definite form. Glauco- 

 phane is also very irregularly distributed throughout the exposure. 

 Under the microscope the groundmass is seen to be largely com- 

 posed of closely packed little columns of omphacite. Much of it 

 shows wavy extinction, as if the rock had been subjected to crushing. 

 This is also indicated by the badly fractured condition of the larger 

 garnets. Part of the green groundmass is probably the grass-green 

 hornblende, smaragdite, although rather' indefinite results were 

 obtained in measuring extinction angles. , Irregular light-colored 

 sphene granules are scattered freely through the shdes. A mineral 

 with lighter color than the omphacite shows some pleochroism and 

 very low interference colors. It was determined as chlorite, and is 

 probably derived from the hornblende or the augite. In places it 

 shows a bright Prussian blue interference color — according to Rosen- 

 busch, a characteristic of the pennine division of chlorites. Glauco- 

 phane is plentiful in some of the shdes and in a few instances gave an 



