GREENSTONE SCHISTS IN SAN JUAN MOUNTAINS 503 



of the lighter sihcates; what feldspar there is has evidently been much 

 altered. The most significant features of the rocks in different 

 parts of the complex are the variations in texture and structure which 

 are conspicuous in the field and still more so when the rocks are exam- 

 ined microscopically. As will be shown later, these are due partly 

 to original textural differences in the rocks themselves, and in part 

 to their dynamic metamorphism that has produced, in some instances, 

 finely laminated schists in which all traces of original structure have 

 been obliterated. 



In the field and laboratory two distinct kinds of massive rocks 

 have been recognized, and transitions between them and the schists 

 may be followed in many cases. Rocks of the first kind are granular 

 and of a medium texture. The second consist of porphyries with 

 phenocrysts of eldspar and hornblende in a very fine-grained ground- 

 mass, or are extremely dense, without phenocrysts and resemble the 

 finer-grained diabases. A number of rocks of intermediate textures 

 have been found, but the two groups may be considered as fairly well 

 defined for purposes of description. 



Massive granular greenstone or metagabhro. — Occurrences of 

 strictly granular rocks are not numerous. The best examples have 

 been found in a restricted area which includes Irving Peak and its 

 southwestern flanks near the northern limits of the complex. The 

 texture of these rocks is practically the same as that of many medium- 

 grained gabbros, the only differences being that laths of plagioclase 

 cannot be made out, and that the dark amphibole, although it appears 

 to be in the form of blades or lath-shaped crystals, is, in reahty, 

 fibrous. The average rock is moderately coarse, even-grained, but 

 specimens from the summit of Irving Peak show a tendency toward 

 the formation of fine-grained segregations richer in hornblende. On 

 fresh surfaces the color is a dark bluish- to greenish-gray. 



A microscopical examination shows at once that the rocks are 

 more or less metamorphosed. The chief constituents are horn- 

 blende, plagioclase, and generally a very Httle biotite and magnetite. 

 The plagioclase is seldom fresh enough to permit of an exact deter- 

 mination of its character, but the large extinction angles indicate 

 labradorite. The feldspar and hornblende are present in about 

 equal quantities in large irregular patches rather than grains. The 



