832 DESCRIPTIVE GEOLOGY. 



nized augite and triclinic feldspar, together with occasional though rare 

 grains of quartz. The habit of the rock, and its position inferior to the 

 trachytes, as well as its close resemblance to the decomposed forms of 

 augite-propylite at Silver Mountain in California, has determined its refer- 

 ence provisionally to propylite, and it is so designated on the geological 

 sheets. It is proper to state, however, that augite-propylite is a somewhat 

 rare occurrence, and the rock may prove to be geologically more closely 

 allied to the porphyry described from the upper end of the canon, and to 

 the melaphyr in Berkshire Canon. There are but two points discernible in 

 its geological relations: one, that it is younger than the diorite bodies of 

 the canon ; the other, that it is older than the Tertiary volcanic series, since 

 the trachytes, rhyolites, and basalts all overlie it. 



About 4 miles above Wadsworth, directly on the river-bank, in an 

 exposure made by a railroad- cut, thefe is a body of quartzose diorite, which 

 underlies the propylite. Under the microscope, these diorites have been 

 shown by Professor Zirkel to contain plagioclase, but nearly no orthoclase, 

 dark-green hornblende, quartz, apatite, biotite, and magnetic iron. Asso- 

 ciated with this is a diorite-porphyry, consisting of a fine groundmass, with 

 imbedded crystals of plagioclase and hornblende. A similar outcrop occurs 

 near the head of the canon, just south of Sheep Corral Canon, and about 

 3 miles north of the Truckee River. This latter diorite is a little more 

 quartzose than in the first-named locality. It is overlaid by the Sheep 

 Corral Canon trachytes, and the basalt mass to the south. This basalt field 

 extends from the diorite outcrop down nearly to the railroad, and is essen- 

 tially a part of the field lying directly west of it. These two outcrops of 

 diorite, each about a mile in extent, together with the porphyry at the 

 upper end of the canon, probably form the oldest rocks of the region ; but, 

 owing to their wide separation, there is no means of judging of their relative 

 age. Taken together, they probably represent all that is left in the canon 

 of the original Virginia Range prior to the Tertiary volcanic period. 



Overlying the decomposed propylite mass of the Lower Truckee Canon 

 upxDn its south flank, which is in turn concealed by basalt, is a rather thin 

 bed of a light-colored trachyte, characterized both by individualized crys- 

 tals and by glassy matter; sanidin, liornblende, and biotite appearing promi- 



