CHARACTER AND COMPOSITION OF TIIK ORANITE 05 



})lates, with good ciystallograpliic boundaries, and intimately associated 

 with niuscovite when present. A part of the biotite is invariably par- 

 tially altered to chlorite. Muscovite varies in quantit}', and is always 

 inferior in amount to the biotite. Epidote is somewhat abundant as a 

 secondary constituent in some of the granites. The remaining micro- 

 scopic accessories present show the usual characteristic features. 



The first decided change in the weathering of the massive granites, in 

 which an almost complete change in a{)pearance from the perfectly fresh 

 granite is indicated , is represented by a hard and firm, dull grayish granitic 

 mass, tinged from a faint to a highly ferruginous rusty brown color. It 

 varies in tenacity from the firm, hard rock to specimens which readily 

 crumble under the gentlest })ressure. Aside from change of color and 

 lack of luster in the component minerals, the rock has lost its compact, 

 close grained texture, and the partially changed granite presents instead 

 a somewhat spongy or loose texture in appearance, in which the indi- 

 vidual mineral grains are slightly forced apart from each other. The 

 feldspars are of a decided white color, perfectly opaque to the naked eye, 

 and have commenced to split along the cleavage planes. Under the 

 lens, the feldspars show comparativel}'^ fresh luster in places ; the biotite 

 appears somewhat leached from hydration and oxidation, and the adja- 

 cent areas are slightl}'' discolored from the hydrous iron oxide derived 

 therefrom. In the center of the biotite plates the luster on the cleavage 

 faces is as brilliant and the color as strong as that in the fresh rock. 

 Thin-sections made from hand specimens representing this stage in the 

 weathered rock show a marked yellow color to the naked e3^e, indicative 

 of the iron oxide staining derived'from the decomposition of the biotite. 



A microscopic examination of the thin-sections confirms the above 

 macroscopic description. Under the microscope the feldspars appear 

 somewhat cloudy and opaque, but otherwise are comparatively fresh 

 looking. The plagioclase felds})ars are more altered, as a rule, than the 

 ])otash varieties. The alteration takes place mostly along the twinning 

 planes in the plagioclase, and mainly along the lines of cleavage in the 

 potash feldspars. The only visible change in the quartz is a splitting 

 up along the innumerable fracture lines subsequently induced through 

 dynamic action. The quartz and feldspar areas, particularly along the 

 lines of cleavage and breakage, are stained yellow by the hydrous iron 

 oxide derived from the partial decomposition of the biotite. 



The biotite appears in part perfectly fresh, but a large i)roportion of 

 this mineral has lost its cleavage and optical properties, and is replaced 

 by an amorphous yellow to brown colored inass, with the surrounding 

 areas partially discolored from the ferruginous constituent. In still other 

 shreds of the biotite the edges and borders are frayed out and leached, 



