SUMMARY. 371 



In the decomposition of the feldspars, the first stage appears to be the 

 formation of calcite. This sometimes leaches out, leaving small irregular 

 cavities, and these cavities are not infrequently filled with liquid, some- 

 times carrying a bubble, which is commonly stationary, but occasionally 

 active. Thus secondary liquid inclusions are formed, which may mislead 

 in the diagnosis of a rock. Primary liquid inclusions are either more or 

 less perfect negative crystals or vesicular bodies The vesicles often assume 

 strange forms through pressure, such as are often observed in air-bubbles 

 in the balsam of a slide, but their outlines are composed of smooth curves. 

 The secondary fluid inclusions are bounded by jagged lines. Inclusions of 

 this kind are never met with unaccompanied by other evidences of decom- 

 position, and thus are abundant in the altered outer crust of andesite masses, 

 the inner portions of which show none of them. There is every reason to 

 suppose that such secondary inclusions would form in older rocks, and it is 

 believed that many of them have been detected in the pre-Tertiary erup- 

 tives of the District; but in the older rocks their secondary character can 

 only be suspected, not proved. 



Kaolin possesses so few characteristic optical properties that it is not 

 recognized with ease or certainty under the microscope. No kaolin has been 

 identified in the Washoe rocks, and while it is by no means asserted that 

 they contain none, it seems hardly possible that, had it formed a prominent 

 constituent, it would have escaped observation. The presence of enormous 

 masses of "clay" on the Comstock does not prove the existence of much 

 kaolin, for the so-called clays of veins are largely attrition mixtures. 



An increase in volume appears to accompany the decomposition of the 

 Washoe rocks. This is perceptible where dense masses, such as the more 

 compact andesites, are subjected to the process. Angular blocks are then 

 converted into a series of concentric shells of comparatively soft matter, 

 which approach the spheroidal shape more and more as the diameter dimin- 

 ishes.^ Often a nodule of undecomposed rock is found at the center, and 

 such masses afford the very best opportunity for studying the macroscopical 

 appearances resulting from degeneration. When the attacked mass is large, 



' Prof. E. Pnmpelly lius described the course of decomposition .almost iu the same words, in his 

 paper "On the Relation of Secular Rock-disintegration to Loess, etc." (Amer. Journ. XVII., 1879, 136.) I 

 did not happen to see Professor Puinpelly's paper until after this passage was written. 



