THE ROCKS or THE WASHOE DISTIMCT. 59 



an originally porous texture. The breccias remain hard and tough until 

 every mineral has been subjected to comi^lete altei-ation. There is nnu-h 

 evidence and every analogy to show that this decomposition proceeds from 

 external surfaces, cracks, and fissures tovvai-d the centers of blocks or masses. 

 Very frequently where cuts have exposed altered rocks, blocks of small size 

 may be seen, which consist of concentric shells of loose decomposed rock- 

 substance, and still contain kernels of fresh andesite. Tlie size of the blocks 

 is, of course, a matter of accident, and sometimes extensive masses decom- 

 pose only from their external surfaces. When this is the case erosion 

 often acts more rapidlj^ than decomposition and, as the decomposed r<H-k 

 is comparatively soft, masses of the fresh ande-site are frequently left staiidiiK- 

 above the general level. The fresh rock thus exposed has the appearance 

 of a cropping of a younger eruption penetrating and overlying an older and 

 different one; and this appearance is heightened by the weathering of the 

 pseudo cropping which, as already explained, results in a mass of reddish- 

 brown fragments quite unlike the product of alteration beneath the surface. 

 The andesite which had decomposed under ground used to be regarded as 

 propylite, but careful examination of exposed masses of andesite such as 

 those described, shows that a transition into the propylitic form may always 

 be followed out at their base As the course of the decomposition is depend- 

 ent on the presence of accidental fissures and, no doubt, on the texture of 

 the rock, the form of the residual masses of undeconiposed andesite is fan- 

 tastically various, sometimes resembling dikes, again assuming the shape of 

 domes and cones. 



Distinctive characteristics. — Hoi'ul jleude-audesites are distinguishal)le from the 

 augite-andesites when fresh by the presence of abundant porphyritic lioi-n- 

 blende crystals and by the luster, which in the augitic rocks is resinous. 

 From the porphyritic diorites they are distinguishable macroscopicali}- by 

 a lack of the granular structure, which the older rock commonly shows. 

 In the propylitic stage of decomposition the three rocks are almost indis- 

 tinguishable. 



Speculation on "black border." — Somc of the Washoe audcsitcs sccm capable of 

 throwing light on the conditions under which the black liorder fiirms aljout 

 hornblende crystals. In slides from difierent part.s of the District two con- 



