GREENSTONE SCHISTS IN SAN JUAN MOUNTAINS 505 



Between these altered massive rocks and the finely laminated 

 schists, a series of rocks may be found which illustrates the various 

 stages of dynamic metamorphism, and satisfactorily proves the close 

 relation of the schists and massive greenstones. The bending and 

 fracturing of the feldspars is followed by more complete crushing, 

 and the hornblende is broken up and redistributed in small parallel 

 blades through the rock. In extreme cases recrystallization has 

 probably taken place, and the minerals in such rocks are, as a rule, 

 much fresher than in many of the less mashed varieties. 



Greenstone schist. — The more or less completely schistose rocks 

 which make up the major part of the Irving series differ but little 

 from many of the schists of the Archean. They are fine-grained 

 and well laminated and of a dark greenish-gray color. The micro- 

 scope shows them to be made up of pleochroic green hornblende in 

 excess of finely granulated feldspar, and usually biotite, a little quartz, 

 and magnetite. The parallel arrangement of the minerals, especially 

 of the blades of hornblende and biotite, is very striking. Feldspar 

 sometimes occurs merely as interstitial grains between the dark sili- 

 cates, but usually it is present as a fine mosaic in long-drawn-out 

 lenses or bands. In some cases chlorite has completely replaced 

 hornblende, and muscovite has been developed at the expense of 

 part of the feldspar. 



Siliceous schists.— At a number of localities within the greenstone 

 area schists and gneisses of a much less basic character have been 

 found. They are light gray or nearly white in color, the dark silicate 

 is biotite, and the feldspar, when recognizable, orthoclase and micro- 

 cline, with only small quantities of plagioclase. Quartz is abundant, 

 and the microscope shows that magnetite, muscovite, and rarely 

 augite or hornblende, may occur as accessories. In all cases the 

 mashing has been sufficient to destroy original textures and develop 

 an excellent schistosity or lamination. Except in one or two doubtful 

 cases, to be noted later, these rocks appear to have been formerly 

 intruded into the greenstones as granites, and subsequently to have 

 shared with the older rocks in the mashing and deformation of the 

 region. 



The exceptions just mentioned are some unusually siliceous 

 rocks whose relations to the greenstones are not altogether clear. 



