STRATA UNDERLYING BASALT—EROSION BY THE SAN JOAQUIN. 
23 
hills along the Merced and other rivers. They will probably he found to he of recent tertiary 
age. It is evident, from the fact of superposition, that the basalt is more recent in its origin. 
In these extended igneous layers we have a remarkable and highly interesting instance of an 
immense overflow of molten rock, spreading itself out in a broad plain not less than fifteen or 
twenty miles in length and breath, and probably of much greater extent. Its compact and 
crystalline character, and the almost entire absence of that amygdaloidal, porous, and scoria- 
ceous character peculiar to surface lava streams, indicate that it was subjected to pressure at the 
time of its overflow, and point to its submarine origin. 
It may be difficult for vhose who are not familiar with the many examples of submarine over¬ 
flows of lava, to conceive how this bed of rock could have spread itself out so uniformly over 
such a wide area, where we now find deep valleys and steep mountains, and scarcely a trace of 
any horizontal sedimentary rocks. This difficulty vanishes when due consideration is given to 
the ever-active and powerful denuding force of rivers and floods, and it is seen that the deep 
valleys and ravines that intersect this lava-plain have been excavated by their action. 
The stratified sedimentary formations found lying horizontally under this lava indicate that 
the whole surface of the country was once overlaid by a continuous deposit of sediments in hori¬ 
zontal strata, which filled up all the valleys and inequalities in the original granitic surface, 
and thus produced a nearly level plain or slope of gentle inclination over what was previously 
a surface of rugged granite ridges. The inequalities of surface having been thus filled up by 
deposition, the overflow of fluid rock took place and spread out over the plain in an even layer. 
The stratified deposits are probably of marine origin, and may have been deposited at the bot¬ 
tom of a deep and quiet sea. 
It was probably the case that the granite ridges were so high in many places that their tops 
were scarcely covered, and that some stood out above the general surface of the sedimentary 
layers. Thus, in some places, we find the lava in contact with granite ; and again, it is under¬ 
laid by the sediments. It will probably be found that there are many places in this unexplored 
expanse of rock where the summits of the old granite ridges stand out above its surface like 
islands. Wherever this basaltic covering has been removed by denudation or otherwise, all 
the soft sediments, composed of exceedingly light materials, have been rapidly torn away by 
floods, and they are not now to be found except in some protected angle, and under the shelter 
of the projecting edge of the lava. 
The existence of these soft strata, filling up all the inequalities of the ancient granite surface, 
and laying the foundation for this igneous overflow, has proved the main cause of its speedy 
downfall and decay. 
The foundations of the rocky plain have been of sand, and wherever they have been exposed 
they have crumbled and been washed away, leaving great overhanging cliffs of lava to break 
off in successive masses by their own weight. Wherever the lava rests upon granite, the degra¬ 
dation of the plain appears to have nearly ceased, or to progress but slowly. 
It is not impossible that this lava-plain was once overlaid by sediments similar to those now 
found under it. No remnants of such formations were found. 
It should be remembered, that at the time when the present Coast Mountains were partly 
submerged, and held the same relation to the slope of the Sierra Nevada that is now sustained 
by the Farallones and other islands to the present coast, the plain of basalt was still partly 
beneath the depths of the primeval ocean; and that, during the period of elevation, the surf 
must have rolled for ages at the base of its dark cliffs, excavating deep caverns in the under- 
