358 RULIFF S. HOLWAY 



eclogite margasite is very plentiful and has been fully described by 

 Ransome, as already cited. White mica, probably paragonite, 

 occurs in the San Jose eclogite and in that from Sonoma county. 



Titanite, or sphene, is thickly distributed in some of the slides in 

 very minute grains. It is a very light brown color and pleochroic 

 in tints of that color. Rutile in large crystals 10-15™™ i^ diameter 

 is found only at San Martin, but it is found somewhat less freely than 

 sphene in the shape of small irregular grains in nearly all the slides. 

 It is yellowish to reddish-brown and somewhat pleochroic, and has 

 very high relief. 



Epidote is rather irregular in its occurrence. When found, it 

 exhibits lower interference colors than those usually described. 

 Ransome^ thinks that this is accounted for by a smaller proportion of 

 iron, and that in chemical constitution the epidote may grade over 

 into zoisite. Zoisite and cyanite were seldom found. Pyrite appears 

 occasionally, but not so frequently as was expected from the descrip- 

 tion of the European eclogites. 



All of the above minerals are possibly secondary in their occur- 

 rence in eclogites. The occasional feldspar and quartz may be the 

 only minerals remaining unchanged from the original rock. The 

 garnets and the epidote have probably taken up the line of the original 

 feldspar, while the soda is to be found in the glaucophane or in the 

 pargasite variety of hornblende. 



While the use of the term "eclogite" is now fairly definite, there is 

 still a question of some limitations in its application. In constitution 

 the rock must contain garnets in a matrix of omphacite, glaucophane, 

 or hornblende, or of some mixture of these minerals. Hezner evi- 

 dently would insist that there must be omphacite, for even with about 

 equal proportions of hornblende he drops the term "eclogite" and 

 uses "eclogite-amphibolite." The common accessory minerals are 

 sphene, rutile, epidote, apatite, zoisite, cyanite, feldspar, and quartz. 

 While some metamorphosed sedimentary rocks approach eclogites 

 in composition, the tendency seems to be to restrict the term to rocks 

 that are clearly of igneous derivation. 



RULIFF S. HOLWAY. 



Berkeley, Calif. 



^ Op. cit , p. 310. 



