OCCURRENCE AND SUCCESSION OF ROCKS. 203 



allelism of structure so frequently referred to. Even the sinuous form of 

 the CoMsTocK is almost exactly reproduced in the Occidental lode. Bedded 

 flows aggregating- over a mile in thickness could never have resulted in so 

 nearly perfect a parallelism. 



Later hornbiende-andesite. — The Sutro Tumiel scction shows a fourth vcry steep 

 contact between augite-andesite and younger hornbiende-andesite; but the 

 eastern portion of the former, though covered for the most part, did not 

 sink in the younger rock below the level of the tunnel, and even reaches 

 the surface near the mouth of the adit. The manner in which the portions 

 of diabase and earlier hornbiende-andesite which lay to the east of the 

 masses now in place disappeared, is a matter of speculation; the Sutro 

 Tunnel section shows that the corresponding area of augite-andesite really 

 sank into the later hornbiende-andesite. Had it settled a few hundred feet 

 farther, it would have left as little trace behind it as did the earlier rocks. 

 So far as the Washoe District is concerned, however, the eruption of later 

 hornbiende-andesite was probably less violent and less voluminous than 

 that of either of the preceding andesites, and was therefore not so likely to 

 bury the east country to a great depth. Above ground, instead of lying 

 on a curved surface reducible to an original plain, it forms a range of 

 mountains extending to the north far beyond the limits of the map. These 

 do not appear to have suffered greatly from erosion, for even near the sum- 

 mits they are largely composed of tufa and tufaceous breccia, which could 

 offer little resistance to water currents. It does not appear to me that the 

 existence of this range in its present form is compatible with the suppo- 

 sition that a large area of the same rock has been removed by erosion. 

 Making allowance for faulting, the older and firmer rocks have been worn 

 down to a tolerably smooth and uniform surface, upon which the present 

 younger hornbiende-andesite range lies in rugged masses. Had the older 

 andesites and the diorite been cut away after the formation of these hills, the 

 latter must have suffered at least as much as the older rocks. 



Evidence of slight erosion. — There is uo evideucc that they have done so; on 

 the contrary, if the contours of the map within the area laid down as younger 

 hornbiende-andesite are examined, it will be seen that these are not such as 

 commonly result from deep erosion. Compare, for example, the steep slopes 



