74 QncKsn.vEK deposits of the pacific slope. 



rocks by noting tlie minerals which result from the metamorphic processes 

 one by one, leaving for subsequent discussion the various combinations in 

 which they occur. 



Biotite. — When foils of biotite are compressed into zigzag outlines by tlie 

 pressure of adjacent clastic grains the mineral is evidently allothigenetic. 

 In some other cases tliere is a lack of decisive proof as to the origin of the 

 biotite, but there are also occurrences which can only be interpreted as au- 

 thigenetic. The authigenetic biotite scales are sharper in outline than the 

 allothigenetic foils, and are usually of a li^'ht, clear, cliestnut-brown color. 

 In cross-section they are often seen to be undulous, but do not form broken 

 lines like clastic foils. They are frequently embedded in recrystallizing 

 feldspar grains. The quantity of this mica detected is small, and it seems 

 probable that when formed it readily passes over into white mica by epi- 

 genesis. In one glaucophane schist from New Idria there is a great abun- 

 dance of fine, nearly uniaxial biotite. 



Muscovite — The epigenetic formation of Avhite mica from biotite and from 

 feldspar has long been recognized. In the recrystallizing sandstones of the 

 Coast Ranges white mica is rather rare as an indubitabl}^ allothigenetic com- 

 ponent, but is very common as an alteration product of brown mica. It also 

 appears to form in the cementing mass of tine detritus and deposits between 

 the clastic grains of sandstones; but, while the occurrences and the analogies 

 are such as to warrant an opinion that such foils of wdiite mica are authi- 

 genetic or epigenetic on authigenetic biotite, it can hardly be demonstrated 

 that this material is not allothigenetic. In the more altered rocks it is seen 

 forming in disintegrating feldspar grains and it is an important constituent 

 of the glaucophane schists. Where it can be separated in foils it is found 

 that the angle of the optical axes is large. 



Augite. — Though a careful watch has been kept for rhombic pyroxene, 

 none has thus far been detected in any of the metamorphic rocks. In the 

 rocks which retain an unmistakably clastic structure augite is rare, a fact 

 which appears to be due to the tendency of the mineral to decomposition 

 when the structure of the rock in which it exists is sufficiently open to permit 

 of the free percolation of solutions. There are a few examples, however, 

 which leave no doubt as to the fact of tlic formation of aurrltein sandstones 



