PETHOGHAIMIY—SANIDIN TRACHYTE. 52 1 



quite traiisparoiit and iiuludo a great many nncrolites, and the needles (»f 

 the j^roundniass press into the crystal along the border in such quantities as 

 to explain perhaps the formation of the rock. The solid mass consists for the 

 most part of clear and transparent sanidin, which, however, has been pierced 

 through and througii and filled up with vast numbers of yellowish-green 

 hornblende needles, leaving an occasional crystal free, which now behaves in 

 polarized light in a similar manner to the mass in the interstices between the 

 microlites. The oreen color of the rock results from these microlites. A similar 

 structure, but with much larger hornblende crystals, can be observed in the 

 rocks [123] and [15i)-lG2], the fluid-like structure in the latter being also 

 much more marked. In the slide now under examination, this structure is 

 indistinct, being best seen around a sanidin crystal as if the force of the crys- 

 tallization had pushed the microlites closer together. There are a few quite 

 large, yellowish-gi'een hornblendes, which are dichroitic, although not very 

 strongly so, and show some traces of cleavage lines. They include grains 

 of magnetite and one in particular contains a piece of magnetite about one- 

 third of its size, which confonns to the shape of the hornblende as if it was 

 the result of a secondary formation The section of [124] has a similar 

 occurrence of hornblende and magnetite. The hornblende has also brown 

 spots through it, the beginning of alteration. Magnetite in small grains is 

 present, but not very abundantly, and must not be confounded with very 

 minute black biotite needles. 



The greenish color of the microlitic groundmass can be seen to the 

 best advantage with reflected light. 



The sanidin-trachyte [178] from the top of Warren Peak is a charac- 

 teristic looking rock, with enormous sanidin crystals in a light-grayish 

 groundmass, with iron-brown spots in it. Some of the crystals are from 

 one to two inches in length. In the section, the rock appears to be very porous 

 and to consist of a fine groundmass, in which are the large crystals, with 

 some hydrated oxide of iron and nearly decomposed biotite The large 

 sanidins present no remarkable appearances, while the feathery or fibrous- 

 microlitic groundmass has been previously described as characteristic of 

 trachyte. The biotite has become so nearly opaque and brown as to lose 

 all of its peculiarities, except the rough, hexagonal outline. But little mag- 



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