298 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



Alteration.— The rocks of the bedrock complex are all more or less 

 altered. The silicious clastic rocks have all become quartzite, with no 

 visible clastic structure. The purer limestones are all crystalline. The 

 gypsum is all crystalline granular, although this may perhaps be an 

 original structure. The argillaceous rocks seem to have suffered altera- 

 tion, varying in intensity from place to place; .but the results may sub- 

 sequently be shown to be due to variations in original composition. 

 Some of these rocks appear to be shales, finely laminated parallel to the 

 bedding planes, often very friable and not notably altered in texture ; 

 but others have been distinctly metamorphosed by dynamic agencies 

 and are now well developed slates. Slates are exposed in Muttleberry 

 canyon near the spring, but still better in a canyon some three or four 

 miles north of Muttleberry. The rock is quite hard, cleaves into large 

 slabs, shows the cleavage and original laminations to be discordant, and' 

 has a luster on the cleavage face denoting a certain amount of recrystal- 

 lization. Another peculiar slate, well exposed about a mile east of the 

 gypsum deposit, breaks up into long thin rods up to 8 or 10 inches in 

 length and rarely as much as a quarter of an inch through. 



Another form of alteration is especially noticeable in the impure lime- 

 stones. These, during the folding of the series, have been broken through 

 by innumerable small fractures, which ramify into a fine-meshed network 

 in the rock. The fractures have been subsequently filled with calcite, 

 forming a network of white veinlets. The effect is striking when the 

 white secondary calcite occurs in black limestone. The pure limestones 

 do not show this brecciation. 



This is not the only t3^pe of vein that intersects the bedrock com- 

 plex. The slates and even the limestones are frequently traversed by 

 quartz veins. These vary from thin stringers up to veins 5 or 6 and oc- 

 casionally more inches across. Such veins have been noted at many 

 places in the range, and while they sometimes contain considerable 

 pyrite they are often of " barren " quartz. Nowhere in the southern 

 range have they proved of any particular value, though in the Star Peak 

 range there are many well known mining camps. 



Barite veins have been found by the writer at two localities — one on 

 the west slope, 12 miles south of Lovelock, and the other on the east 

 slope, directly east of the same town. Fluorite was also found at the 

 former locality. 



Geomorphic relationships. — The effect of the bedrock complex on the 

 topographic forms is frequently decided, although not great. Weather- 

 ing generally produces rather smooth rounded forms, as is well shown in 

 plate 17, figure 1. In other words, residual boulders, cliffs, crags, or other 

 prominent forms are not generally produced. This applies particularly 



