46 
W. Pengelly on the Bed Sandstones, 
require us to ascribe an enormous thickness to the Red Rocks of 
our county, though we must hesitate to claim for them the results 
of the calculation based on the data which they furnish. 
It must not be forgotten, moreover, that we now see but a 
remnant of the formation. Outliers and other phenomena indi- 
cate that it formerly covered a much greater area ; and when we 
observe how deep are the valleys which denudation has formed in 
it, w^e cannot doubt that the hills which bound them have lost very 
much of their altitude. Enormous as is its volume now, it is but 
a fraction of what it once was. 
Since it is known that in 1 783 the Volcano of Skaptar Yokul, 
in Iceland, poured forth such floods of lava that " one of the 
streams was fifty miles long and fifteen wide in some places ; and 
another forty miles long with an occasional width of seven miles ; 
each of them having an average depth of 100 feet, increasing here 
and there to 500 or 600,"* it becomes necessary to keep in view the 
origin, as well as the volume, of a formation when speculating on 
its chronological value. Now every cubic inch of the rocks we 
have been considering is of mechanical origin. No one, however 
slender his geological information, can contemplate a bed of 
ordinary conglomerate without a conviction that it is a consequence 
and a proof of the destruction of at least an equal amount of pre- 
existent rock. Each pebble it contains is evidence of the successive 
processes of denudation, attrition, transportation, and deposition. 
It is not certain that a piece of fine sandstone so readily impresses 
the unscientific observer; nevertheless, the only difference between 
the two rocks is merely that of between coarse and fine. The 
particles composing the sandstone must have experienced the same 
mutations as the pebbles forming the conglomerates; but with this 
difference : in the former, the grinding down of the fragments of 
the parent rock was completed, in the latter it was but begun ; in 
the one case the entire mass was reduced to fine sand, in the other 
only the angularities were removed. All other things being the 
♦ Jukes' '* School Manual of Geology," page IG, (1863). 
