TPtUCKEE CASTON. 829 



several miles back from the river, and falling away in broad, uneven steps. 

 It is an unusually coarse-grained rock. It contains little or no olivine, but 

 possesses a large proportion of a curious dark-green, fibrous substance, resem- 

 bling, as Professor Zirkel suggests, the characteristic anamesite of Stein- 

 heim, and, like the latter, shovs^s, under the microscope, the presence of apa- 

 tite. The basalts on the opposite side of the canon extend northward for 

 many miles, and in general differ from the others by presenting a finer text- 

 ure, and by the presence of passages of very vesicular rock, in which the 

 pores often reach a half -inch in diameter. Mineralogically, the rock is rela- 

 tively rich in olivine, but, in the specimens examined, the apatite crystals 

 found on the opposite side of the river are wanting. 



It is evident that the Truckee Canon, for the first 4 miles of its descent, 

 represents something more than a valley of erosion, or at least than that 

 modern erosion which has existed since the basaltic period. The evidence 

 of narrow, limited flows down the lateral walls of the north flank of the 

 valley is very clear, and the entire surface of the broad, open area from the 

 narrows, near the above-described porphyry outcrop, down nearly to Clark's 

 Station, shows a topography altogether dependent on the original basaltic 

 surface. Over the fields, as one ascends from the valley-bottom, there is seen 

 but a slight accumulation of soil, much of the rock being as bare as at 

 the time of its original flow. The immediate river-valley here, as above, 

 is occupied by Quaternary sands and gravels, overlaid by the present 

 accumulations from the valley- walls. 



About 3 miles above Clark's, on the north side of the valley, occurs 

 an isolated body of trachyte, overflowed by basalt on the west, and by rhyo- 

 lite on the east. 



Antoine's Canon, which joins the Truckee Valley from the north, bring- 

 ing down the drainage of Spanish Peak, is cut altogether in rhyolite. This 

 rock makes a large display, having its culminating point on Spanish Peak, 

 and descending thence in all directions. Its central body is a fine-grained 

 felsitic mass, showing but few individualized crystals of feldspar. Speci- 

 mens in the collection from here afford characteristic instances of the finely 

 striped and laminated rhyolitic structure, the lines and bands appearing 

 alternately of a pale-brown and a pale-lilac color. Mineralogically, the rock 



