248 GEOLOGY OF THE HIGH PLATEAUS. 



still be discovered upon weathered surfaces, though, in fresh fractures they 

 appear to have gained an aspect very nearly homogeneous with the general 

 mass. This phenomenon of the gradual vanishment of pebbles is not con- 

 fined to the tufas, but is frequently seen in the conglomerates, some of 

 which have been greatly altered and converted into a hard semi-crystalline 

 rock strongly resembling andesite and hornblendic trachyte. Moreover, 

 the inferior boundary of the larger sheet is indefinite in many places, and 

 near the fault it appears to have passed lower down and involved beds 

 which are not so much affected farther up the canon. The lines of bed- 

 ding near the fault are nearly obliterated, and the thickness of the lava-like 

 mass has greatly increased. I entertain very little doubt that the sheet is 

 not a lava, either contemporaneous or intrusive, but is a metamorphosed 

 tufaceous deposit. 



Farther up the East, Fork Canon, upon the north side, stands an iso- 

 lated mass, consisting of phonolite, represented in Heliotype No. XL It is 

 a hill about 1,400 feet high, with steep flanks, covered with talus. Near 

 the summit the cleavage of the rock in vertical planes is exhibited with 

 clearness. Upon closer inspection a secondary cleavage, perpendicular to 

 the foregoing, is also disclosed, and the viscous vitreous character of the 

 lava is very conspicuous. Under the microscope it discloses very few 

 crystals, and these are very small, consisting of nephelin. No feldspar was 

 detected. The specimens brought home, though fair in appearance, proved 

 to be much weathered and hardly suitable for microscopic or chemical 

 investigation. The plateau mass around this hill was much eroded, and 

 the eruption of the phonolite appears to have occurred after the erosion 

 had far advanced, for it is an isolated mass, and its lavas flow over rugged 

 ridges and ravines upon its northern side. 



GRASS VALLEY. 



Separating the second and third ranges of tabular uplifts is a broad 

 depression, named Grass Valley ; a name which has done great service in 

 the West, for it may be found in every State and Territory. It is properly 

 an appendage of the Sevier Plateau, from the platform of which it has been 



