MAN AND THE LAVA BEDS. 



689 



There are in many parts of California two systems of river- 

 beds, an old and a ?iew. The old belongs to the Tertiary ; the 

 new, to the Quaternary and present. The change took place 

 during the oscillations of the Quaternary. The old river-system 

 is substantially parallel to the present river-system, though in 

 some places the one cuts across the other. It is probable, 

 therefore, that there was but little change in the general direc- 

 tion of the slope, produced by the oscillations of this epoch. 

 These old river-beds are filled with drift-gravel, and often cov- 

 ered with lava-streams. These drift-gravels probably repre- 

 sent the beginning of the Glacial epoch, though Whitney 

 thinks an earlier or Pliocene epoch. The present river-system 

 sometimes cuts across, sometimes runs parallel to, the lava- 

 filled beds of the old river-system, and the beds of the former 

 have in their turn been eroded two thousand to three thousand 

 feet in solid rock. In these also have been accumulated im- 

 mense quantities of gravel and bowlder drift, evidently brought 



Fig. 193.— Lava-stream cut through by rivers ; a, a. basalt : b, b. volcanic ashes ; c, c, ter- 

 tiary ; d, d, cretaceous rocks : R. R. direction of the old river-bed ; R'. R\ sections of 

 the present river-beds. (Le Conte from Whitney.) 



down from the glacial moraines by the swollen rivers of the 

 Cham plain and early Terrace epochs. These facts are illus- 

 trated by Figs. 193 and 194, in which R represents the present 



Fig. 194. — Section across Table Mountain, Tuolumne County. California, b, lava ; G, grav- 

 el ; 8, slate : R. old river-bed ; R', present river-bed. (Le Conte.) 



river-system, in Fig. 193, cutting across, and in Fig. 194 run- 

 ning parallel to, the old system R. 



Although it is impossible to synchronize with certainty 



