GEOLOGICAL FEATURES OF THE TRUCKEE REGION 669 



meadoNA's were often flooded, and temporarily — perhaps for many years at a 

 time — occupied by a lake. At present the river is flowing over volcanic bed- 

 rock at the entrance to the canyon, while the valley is occupied by alluvium 

 and lake beds. The ground-water moving from the west has a level which 

 gradually approaches the surface and appears above it in hollows, producing 

 more or less marshy ground near the Virginia Range front. We have there- 

 fore what is practically an underground lake which, save for percolation into 

 the bedrock, must discharge over the rim at the canyon opening. During 

 periods of considerable water supply from the west the ground-water level 

 rises above the ground and a certain area becomes flooded. Renewed faulting 

 along the range front would quickly produce a "permanent" flooded area. 



The Truckee River channel has occasionally been occupied or displaced by 

 Quaternary basaltic lavas. One flow occurred in the canyon through the Vir- 

 ginia range. It extended clear across the canyon, and the river, hemmed in by 

 hard volcanic walls, was forced to cut through it, and has left it as flat ter- 

 races projecting from the canyon walls. 



About 10 miles west of Reno successive flows came down from the flanks of 

 Peavine mountain and occupied a channel cut in Tertiary lake beds and ande- 

 sites. In this case the river found it easier to cut through the lake beds, and 

 now flows at the base of the basalt, which rises 450 feet in a striking cliff. A 

 long basalt flow, now left as terraces, is to be found higher up the river near 

 Truckee. 



Two sets of hot springs, probably genetically connected with the acute post- 

 Tertiary deformation, are found on the east flank of the Carson range (Steam- 

 boat springs) and about 5 miles west of Reno (Laughtons springs). Several 

 wells sunk along the front of the Carson range have struck hot water, indi- 

 cating other hot springs that do not reach the surface, but feed direct into the 

 ground water. 



The Quaternary river deposits ai-e faulted just west of Reno sometimes as 

 much as 30 feet, and scarps topographically very young are frequently ob- 

 served. In gathering data concerning the San Francisco earthquake of 1906 

 information was obtained concerning small shocks felt distinctly at several 

 towns, but rapidly dying away in distance and so circumscribed that they 

 pointed to recent disturbance along the Virginia mountains. The most active 

 part of the orogenic cycle inaugurated in the early Quaternary is probably 

 passed, but the forces do not seem to have reached yet a condition of equi- 

 librium. 



The writer is indebted to Professor G. J. Young for three of the photo- 

 gi-aphs used in the illustration of this paper. 



A COLLECTION OP FOSSIL FISHES FROM NORTHEASTERN BRAZIL 

 BY J. C. BEAIfNEE AND D. S. JORDAN 



DIATOMS AS A SOURCE OF SILICA IN SEDIMENTARY BEDS 

 BY J. C. BRANNEE 



Adjournment was then taken until afternoon. 



I'VI — Bull. Geol. Soc. Am., Vol. IS. lOdC 



