210 
NOTES OF AN AMERICAN TOUR. 
slabs from the face of the cliffs. The granite presents the 
appearance of having been laminated vertically from some 
cause, and undergoing a process of being split off in succes¬ 
sive slabs of one inch to a foot or more in thickness. In the 
course of the ascent by the zig-zag paths up the cliffs there 
are seen large quantities of these slabs, partially detached 
from the face of the cliffs, some of very large extent, and 
looking ready to fall with the frost of another winter. Now, 
this weathering action is, at the present time, widening 
the valley surely though slowly; and if carried back long 
enough a similar action would have sufficed for the excavation 
of the entire valley from some small commencing fissure ; but 
a difficulty that presents itself to this view is the entire 
absence of any large accumulation of debris as a result of 
the excavation of the entire valley, which is from one to two 
miles in width, and more than half a mile in vertical depth. 
Also, the river running through the valley is, at the present 
time, a quiet stream, not capable of carrying away any con¬ 
siderable quantity of debris. 
In reference to this it has to be noticed that the material 
of the cliffs, granite, is already far gone in decomposition by 
the time it has become sufficiently jointed and loosened to 
allow of its being detached by the weathering action, and will 
not require much further decomposition before it becomes com¬ 
pletely broken up into the condition of the soil of the valley. 
A point of special interest and difficulty in connection 
with this question is the peculiar condition of the Half Dome, 
which is at the upper part of the valley, and is the highest 
point of the surrounding rocks, being nearly one mile vertical 
height from the floor of the valley. There are several 
domes around the valley, consisting of hemispherical eleva¬ 
tions in the granite of large size, and this Half Dome is 
the largest of them, being a quarter of a mile in height 
above the surrounding country and nearly half a mile 
in diameter; and the special feature of it is that the dome 
is cleft vertically, and more than one-tliird of it removed, 
leaving a vertical face towards the valley, but without any 
visible trace being left of the missing portion. It has been 
suggested that the whole valley is the result of volcanic agency, 
and that this dome was split by some upheaval, and the lost 
portion swallowed up in a chasm opened by the upheaval. 
The idea, however, of the weathering action by vertical 
jointing may also be applied to the formation of the Half 
Dome, provided that its structure admits of vertical jointing ; 
but there is a curious circumstance in one of the other domes 
that is accessible for examination, the Sentinel Dome, which 
