MAN AND THE LAVA BEDS. 691 



Quaternary. But, all the facts considered, it seems most prob- 

 able that both the filling of the old river-beds, and their pro- 

 tection by lava, fcook place comparatively rapidly, and were 

 together the closing scene of the Tertiary drama. The deep 

 gravels, therefore, may be placed indifferently in the latest 

 Pliocene or earliest Quaternary. The newer gravels are un- 

 doubtedly Quaternary and recent. Certain it is that the deep 

 placer-gravels are similar in all respects to the Quaternary 

 gravels all over the world, except that, by percolating alkaline 

 waters containing silica, they have been cemented in some 

 cases into grits and conglomerates. This is because they are 

 covered with lava which yields both the alkali and the soluble 

 silica. 



In any case, we have here an admirable illustration of the 

 immensity of geological times. The whole work of cutting 

 the hard slate-rock two thousand feet or more has been done 

 since the lava-flow, and therefore certainly since the beginning 

 of the Quaternary.* 



It will readily be seen that these upper gravels, whether 

 we call them Tertiary or Quaternary, are with reference to 

 the historical period very ancient, though recent if spoken 

 of from a geological point of view. The question of man's 

 antiquity does not turn on the name of the formation ; but 

 upon the reality of the existence of his remains in the upper 

 gravels. Indeed, there does not seem to be any hard-and- 

 fast line of demarkation between the Tertiary formation and 

 the Quaternary, or recent. In making chronological calcula- 

 tions from the vast amount of erosion spoken of in the pre- 

 ceding paragraph, Le Conte warns us, however, to note the 

 prodigious rapidity with which erosion now proceeds in con- 

 nection with hydraulic mining. " In the North Bloomfield 

 mine, the pebble-loaded torrent resulting from the incessant 

 play of the hydraulic jet against the cliff, though working 

 but eight months per year, has cut in four years a channel 

 three feet wide and h'fty feet deep in solid slate." t 



* Le Conte's "Elements of Geology," 1888, pp. 555, 585. 



f " American Journal of Science," March, 1880, vol. cxix, p. 179. 



