VOL. III.] Natural History of the Faj'allones. 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 shoi'e-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 vards 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 84 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 



