92 
GEOLOGY. 
other. It would be difficult to find a place where the cutting power of drifting sand is more 
beautifully and clearly exhibited than it is at this point. The whole surface of the rocks was 
smooth and polished, and even the limestone had a peculiar, rounded and smooth surface, which 
resembled that of partly dissolved crystals, or deliquescent specimens of rock-salt. Long 
parallel grooves, deep enough to receive a lead-pencil, were cut on the surface of the hard and 
homogeneous granite. But the most striking and interesting examples were the effects produced 
on the portions of granite that were composed of large crystals of feldspar , quartz, and tourmaline, 
and also containing small imbedded garnets. These masses of minerals, differing in their 
hardness, were unequally cut away. The feldspar, being the softest, was most rapidly acted 
on, and even the quartz and garnets were unequally abraded, the amount of wear on each 
mineral being in the order of its hardness. The masses of quartz, tourmaline, and garnet thus 
acted as protectors to the portions of feldspar behind and under them, while the exposed parts 
were most rapidly chiseled out by the sharp grains, leaving the harder minerals standing in 
relief, or with the feldspar standing even with their surfaces on the lee side only, thus forming, 
in minature, a kind of tail, similar to the accumulations of earth and stones on the lee side of 
obstructions in a current of water. 
The effects produced on the vertical surfaces of the rock exposed to the wind were, perhaps, the 
most curious and interesting, for here the hard minerals were left standing out in points, the 
softer feldspar being cut out on all sides. Masses of feldspar and quartz thus presented very 
rough and uneven surfaces. A part of the rock, which was detached, is represented in the 
annexed figure—the projecting points being quartz, and the mass of the specimen feldspar ; the 
whole having a beautifully smooth surface. 
ROCK, CUT BY DRIVING SAND. 
Direction of the wind 
Where the feldspar was charged with small garnets, and was directly in front of the wind, a 
very peculiar result was produced ; the garnets were, left standing in relief, mounted on the 
ends of long pedicles of feldspar which had been protected from abrasion under the garnets 
while the surrounding parts were cut away. These pointed masses or needles of feldspar, 
tipped with garnets, stood out from the body of the rock in horizontal lines, pointing, like 
jewelled fingers, in the direction of the prevailing wind. They form, in reality, a perfect index 
of the wind’s direction, recording it with as much accuracy as the oak trees in the vicinity of 
San Francisco, where, if the wind reaches them, they are bent from the perpendicular in one 
direction only, or in some places lie trailed along the ground. All the little points of stone 
pointed westward in the direction of the valley of the pass, to which the wind conforms . 1 
We continued travelling to the southeast, and downwards over the broad slope of the pass, 
1 A more extended notice of these phenomena of drifting sand and sand-polished rocks was communicated to the Ameri¬ 
can Association for the Advancement of Science at Providence, in 1855, and will be found in the Proceedings. See also 
Amer. Jour. Science, second series, vol. xx, September, 1855. 
