BERKSHIEE CASTOK 839 



An analysis of this rock was made in the laboratory of Professor Wiede- 

 mann, in Leipsic, with the following result : 



Silica 64.62 



Alumina 11,70 



Ferrous oxide 8.39 



Lime 8.96 



Magnesia 1.18 



Soda 3.13 



Potassa 1.95 



Phosphoric acid - trace 



Loss by ignition . 1.02 



100.95 



Directly to the north of Sheep Corral Canon is observed the southern 

 end of a bed of massive dacite, which occupies the extreme foot-hills of the 

 range for about a mile, and then trends slightly into the range on a direc- 

 tion about north 20° west. At the point where it trends into the range", the 

 foot-hills are occupied by an outburst of rhyolite, which flanks the range 

 for 8 miles along the Lower Truckee Valley, both rocks being well devel- 

 oped in Berkshire Canon, 3 miles to the northward. 



Berkshire Canon. — About 3^ miles up Berkshire Canon, from the 

 mouth, there occurs a heavy mass of a somewhat peculiar rock, which has 

 been recognized nowhere else within the limits of the Fortieth Parallel Survey, 

 and which has been classed as melaphyr, from its close resemblance in micro- 

 scopical habit to rocks thus designated in Grermany. Of its age here in the 

 Virginia Range nothing is definitely known, except that it underlies the 

 propylite, the oldest of the Tertiary volcanic products. These melaphyrs 

 extend from the head of the canon in a northwest direction about 4 miles, 

 and to the west, north, and south are overlaid by the summit rocks already 

 described, and on the slopes of the range to the eastward are again 

 concealed by propylites and andesites. They are, in general, deeply 

 eroded in transverse ravines prior to the later outflows. In texture, they 

 are extremely varied, passing from a fine-grained, compact, dark-green, or 



