GEOLOGY OF THE SAN JOSE DISTRICT 251 



seen not to be a unit, for erosion has proceeded far enough to 

 level away a considerable space on either side of the main rivers 

 and lesser streams. These level stretches are all situated at the 

 same altitude. The ridges and rounded buttes which stand 

 above them rise to a uniform height so that their tops are parts 

 of a second common plane. 



The mountain slopes of the interior valley in which San Jose 

 is situated are nearly as steep as loose material can stand. The 

 arroyos have commonly made V-shaped cuts with vertical 

 walled trenches at the point of the V. The climatic conditions 

 which affect erosion are peculiar, and deserve to be mentioned. 

 The surrounding country is very dry, almost arid, but the San 

 Carlos Mountains are high enough to be often surrounded by 

 clouds. The dews are commonly heavy, and slight showers 

 may fall once or twice in a fortnight, but the main portion of 

 the annual rain supply comes down in one or two weeks of hard 

 rain. At such times the torrents are immediately concentrated 

 in the mountain gulhes, and twenty minutes after the first drops 

 fall the streams in the center of the valley begin to rise. Many 

 which are dry throughout the greater part of the year then 

 carry three or four feet of water, and their currents are so rapid 

 that they are able to urge along bowlders which are over two 

 feet across. With continued rain the streams may rise five and 

 six feet, and be able to transport a great amount of detrital ma- 

 terials. Fifty thousand tons were removed in this way from a 

 mine dump in twenty-four hours. 



The streams in that portion of the valley which is near the 

 town of San Jose and immediately to the northwest of it, have 

 lowered their beds for i 5 or 20 feet through a rudely stratified 

 deposit of angular and subangular rock fragments. This rep- 

 resents the accumulation at low levels of the loosened material 

 which is everywhere in process of transit down the steep slopes 

 in the central portions of the valley. The andesite, which is the 

 country rock here, is capped by a very scanty cover of soil in 

 most places, while over wide stretches the bed rock comes to 

 the surface. This is covered uniformly by fragments which 

 have weathered away from its superficial portions. These have 



