ECLOGITES IN CALIFORNIA 



349 



nation of the garnets indicates that they are the iron-alumina variety 

 with a considerable amount of calcium. 



Under the microscope the main groundmass of the eclogite is seen 

 to be light-green omphacite, with little, if any, pleochroism. The 

 interference colors are bright, and the extinction angle runs up to 

 40°. The garnets show 

 regular outlines, and are 

 of a pale pink color by 

 transmitted light. Usually 

 they are cracked, with om- 

 phacite filling the cracks. 

 Rutile is present as a seg- 

 regation in the groundmass 

 and as inclusions in the 

 garnets. Sphene of a light 

 brownish color, somewhat 

 pleochroic, and with char- 

 acteristic relief, is plenti- 

 fully distributed in irregular 

 grains. 



The microphotograph in 

 Fig. 2 shows the clear outlines of the garnets. The inclusions in 

 them are omphacite, apatite, rutile, quartz, and sometimes chlorite. 

 The felted groundmass is composed of small columns of omphacite. 

 Many of the slides are practically free from glaucophane, although 

 veins of this mineral are quite plentiful in the exposure. The 

 glaucophane is a somewhat more definite blue than is usually 

 described as steel-blue, and in the slide shows strong pleochro- 

 ism. The color of the a ray is a very pale yellow; the b ray, 

 a purplish violet; the C ray, a clear blue. The extinction angles 

 in several sections were found to be close to 5°. The orientation 

 is that common in glaucophane, with the b axis corresponding 

 to the axis of symmetry and the C axis nearest to c' . In places 

 the glaucophane grades over into a green hornblende. In one of the 

 slides there is an irregular- shaped mineral of a light brown color and 

 cut by intersecting cracks. It is pleochroic in tints of brown. The 

 relief is high, and the interference colors are of the third or fourth 



Fig. 2. — Omphacite eclogite, San Martin. 

 X 24. i=garnet, 2=omphacite, 3=rutile. 



