30 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



4. Andesite. 



Occurrence. — There is one main area of the andesitic 

 rocks with several smaller occurrences. These rocks are 

 all, both macroscopically and microscopically, identical, and 

 undoubtedly indicate an originally continuous area, cover- 

 ing the larger part of the eastern division of the island. 



Ascending the ridge next the ocean, to the south of Mid- 

 dle Ranch Canon, this rock is first met with at an altitude 

 of about 300 feet, where it forms a small patch extending 

 from this point to an elevation of about 500 feet. The next 

 area occurs at an altitude of about i,ioo feet, where the 

 rock not only forms the summit of this part of the ridge but 

 caps, as well, a minor ridge which extends into the adjacent 

 canon. The third occurrence is near the head of this canon. 

 Besides these more definite areas, the soil at a number of 

 places in this region has a purplish tinge, and the general 

 appearance points to a more extended areal distribution of 

 the andesite. Erosion has, however, entirely removed the 

 rock in some places, while in others it has left only the thin- 

 nest coating on the rocks beneath, or the former covering 

 remains simply as a coloring to the soil, in places occupied 

 by other rocks which normally weather to a yellow. 



There is a small area of andesite at the extreme south- 

 eastern end of the island. Here much of the andesite con- 

 tains inclusions in varying amounts, the rock in some places 

 being well filled with this fragmental material, which is de- 

 rived in large part from earlier andesitic flows. 



The main area of the andesite has a general easterly and 

 westerly trend, and extends from the shore on the north 

 side to the lower slopes south of Little Harbor. It reaches 

 an extreme altitude of 2,109 feet. It consists of a series of 

 volcanic flows which present a distinct banding on the face 

 of the cliffs northwest and southeast of Swain's Landing. 

 These bands have a width of four or five feet and upwards. 

 To the east of Swain's Landing they have a dip of io°-i2° 

 toward the Landing, while on the other side they dip in the 

 opposite direction and at an angle of about 3°. Midway 



