via THE ANTIQUITY OF MAN IN NORTH AMERICA 443 



at other times forming detached hills or even mountains of 

 considerable size. These gravel deposits are often covered 

 with a bed of hard basalt or lava, having a generally level 

 but very rugged surface, and hence possessing, when isolated, 

 a very peculiar form, to which the name " table mountain " 

 is often given. These tabular hills are sometimes 1000 or 

 even 1500 feet high, and the basaltic capping varies from fifty 

 to 200 feet thick. The gravels themselves are frequently 

 interstratified with a fine white clay and sometimes with 

 layers of basalt. 



Geological exploration of the district clearly exhibits the 

 origin of this peculiar conformation of the surface. At some 

 remote period the lower lateral valleys of the Sierra Nevada 

 became gradually filled with deposits of gravel brought down 

 from the higher and steeper valleys. During the time this 

 was going on there were numerous volcanic eruptions in the 

 higher parts of the range, sending out great showers of ashes, 

 which formed the beds now consolidated into pipe-clay or 

 cement, while occasional lava streams produced intercalating 

 layers of basalt. After this had gone on for a long period, 

 and the valleys had in many places been filled up with debris 

 to the depth of many hundred feet, there was a final and very 

 violent eruption, causing outflows of lava, which ran down 

 many of the valleys, filled the river beds, and covered up a 

 considerable portion of the gravel deposits. These lava 

 streams, some of which may be now traced for a length of 

 twenty miles, of course flowed down the lower or middle 

 portion of each valley, so that any part of the gravel remain- 

 ing uncovered would be that most remote from the river bed 

 towards one or other side of the valley. This gravel, being 

 now the lowest ground as well as that most easily denuded, 

 would of course be eaten away by the torrents and mark the 

 commencement of new river beds, which thenceforth went 

 on deepening their channels and forming new valleys which 

 undermined and carried away some of the gravel, but always 

 left steep slopes and cliffs wherever the lava flow protected 

 the surface from the action of the rains. Hence it happens 

 that the existing rivers are often in very different directions 

 from the old ones, and sometimes cut across them, and thus 

 isolated table mountains have been left rising up out of the 



