VIII 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 d6bris 
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 
