WEST HUMBOLDT EANGE. 721 



Magnesia 0.23 



Soda 0.79 



Potassa 3.39 



Ignition 0.70 



99.64 



It will be seen that the interstratified chert band is about 16 per cent, 

 richer in silica than the argillite, at the expense of all the bases, except 

 soda, and that the moisture is also very much less. 



The porphyroids, as has been already mentioned, closely resemble in 

 hand-specimens eruptive feldspar-porphyries, and in places also quartz-por- 

 phyries, but the study of their position and petrographical habit in the field 

 indicates a gradual transition by metamorphism from the less altered arena- 

 ceous and argillaceous rOcks up to the compact crypto-crystalline varieties, 

 with a siliceous base, through which are disseminated broken and imperfect 

 crystals of feldspar. It is evident, however, that metamorphism has been 

 carried to such a degree as to have produced a class of rocks quite distinct 

 in habit from those found elsewhere Avithin the region of this exploration. 

 They vary in color from light grayish- white to almost black, with shades 

 of blue, brown, and green. They form dense, tough rocks, M^eathering with 

 an angular surface, and possessing a rough, cherty fracture. In most cases, 

 the texture of the rocks is so fine that they can only be studied to advan- 

 tage in thin sections under the microscope, when they show chiefly quartz 

 and feldspar in a siliceous base. The quartz-grains are frequently rounded, 

 and enclose fragments of the base. Occasional flakes of mica and particles 

 of magnetite occur. No hornblende or augite has been observed. In 

 Coyote Caiion, there is a grayish-black variety, having a texture and lustre 

 like lydite, through which rim roughly parallel lines of narrow gray feld- 

 spar, giving the rock a distinctly banded structure. It appears, upon chem- 

 ical examination, that the dark color is here due to carbon particles, so finely 

 disseminated that even the thin sections are nearly opaque. Much the same 

 banded structure is seen in Buena Vista Canon, the feldspar also occurring 

 in irregular aggregations. There are also small clusters of minute quartz- 

 grains. In the region of Cottonwood Caiion, the porphyroids show a still 



46 D G 



