vou. 11.) Natural History of the Farallones. 145 | 
NOTES ON THE GEOLOGY. 
The islands appear to be the projections of a granite ridge, which 
is elevated about 500 feet above the ocean floor. The granite is 
coarsely crystalline, much fissured and easily decomposed, and is 
mainly similar to that of Point Reyes peninsula. Sugar Loaf 
Rock, the northernmost extremity of the island, is a conglomerate 
of huge rounded boulders, and a 600-foot section of a similar sand- 
stone is exposed at Point Reyes Light, immediately succeeding the 
granite. 
Around the entire island, at an altitude of 50 feet above the pres- 
ent sea-level, an old coast line may be clearly distinguished, marked 
by numerous water-worn caves exactly similar to those now being 
eroded by the waves, and by a wave-cut terrace on the south and 
east sides of the island, which slopes gently from the 50-foot line to 
the water’s edge. It is on this terrace that the dwellings of the 
island are situated. Great Arch Rock was excavated at this level. 
There also appears to have been a short halt between this and 
the present sea-level, for at several points shallow and more recent 
caves are seen 27 feet above the sea, and a dim second terrace 
is shown on the west near the landing. : 
On the west side of Shubrick Point, on the northeast coast of the 
island, the caves marking the three shore-lines are found in nearly 
vertical position. Measurements made here showed the highest to 
be excavated to a distance of 186 feet, the second to about 25, and the 
one at the present sea-level to about 100 feet. It is said that there . 
has been a noticeable elevation of the island in the last thirty 
years. ' 
This uplift of the island reveals another interesting fact: The 
great wave-cut terrace is on the south and east sides, and varies from 
300 yards in width on the former to about 75 on the latter, while the 
north and west shores are steep and precipitous. Now, the con- 
ditions being the same, the greatest amount of wave erosion is on 
the side of the prevailing winds, but, as is well known, the pre- 
vailing winds off the California coast are from the north and 
northwest. The observations of the Signal Service at Cape Men- 
docino and Point Reyes Light show, for monthly prevalence of 
winds, that $4 per cent. are from the north and northwest; even in 
winter, 75 per cent. are from these directions. 
On the south shore of the island, near the dwellings, there are 
