200 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



pyrite is of any importance in throwing light on the origin of the 

 rocks. Its presence suggests that the metamorphism was aided by 

 thermal solutions, which would naturally be alkaline, since only such 

 solutions carry sulphur in any quantity. 



Types of Glaucophane-bearing Rocks of California. 



The glaucophane-bearing rocks of California are varied in 

 character, ranging from massive little altered igneous rocks to thin- 

 bedded schists ; but certain well defined groups may be distin- 

 guished under them. The groups defined below are independent of 

 the genesis, based entirely on the petrographic character, for under 

 most of them are found both rocks of sedimentary origin and rocks 

 that were originally fused magmas. 



I. Eclogites. 



Eclogites were originally defined as massive rocks made up of 

 garnet and omphacite, but since similar rocks often contain actin- 

 olite, glaucophane or other hornblendes as the chief ferro-magnesian 

 mineral, the group has been extended to include these. All the 

 eclogites known in California contain some glaucophane, although 

 some other amphibole is often the predominant mineral. These 

 rocks have been described by R. S. Holway,^ and the principal min- 

 erals in them discussed. The typical variety, those composed of 

 omphacite and garnet, is rare in California, having been described 

 only from two localities. On Coyote Creek, six miles north of San 

 Martin, Santa Clara County, is a massive rock composed of large 

 dodekahedrons of red garnet, long prisms of omphacite, and a little 

 glaucophane and white mica, probably paragonite. Some actinolite 

 occurs in the rock, and abundant rutile in patches. This rock has 

 been analyzed, and the results given on the next page are quoted 

 from Mr. Holway's paper. 



The sample analyzed did not contain any of the segregations of 

 rutile. The estimated mineral constitution is approximately two 

 parts red garnet, two of omphacite, and one of actinolite, glauco- 

 phane, and white mica together. The rock has the composition of a 

 diabase, but the alumina, lime, and magnesia are lower than usual, 



^ " Eclogites in California," Joitr. Gcol, Vol. XII., 1904, pp. 344-358. 



