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



frequently causes them to stand out on the surface of the 

 rock. These inclusions show beyond a doubt that the por- 

 phyrite is the older of the two rocks. 



B. TUFF AND DIATOMACEOUS EARTH. 

 I . Occurrence. 



This material has already been mentioned as occurring 

 intercalated with the andesites of the isthmus region near 

 their upper limits, and forming a single composite bed of 

 considerable thickness. So far as known there is but this 

 one occurrence. Though there are excellent exposures to 

 the east of Isthmus Cove no complete section of the bed 

 was seen, but from the several parts it is estimated to have 

 a thickness of from one hundred to two hundred feet. 



Besides this bed on the island, there is doubtless a consid- 

 erable deposit of similar material just outside of Isthmus 

 Cove, as indicated by the sounding contours. The large 

 scale Coast Survey map of the isthmus emphasizes this, 

 and shows, by mapping in the contours, a more or less con- 

 tinuous submarine ridge, extending out some distance. 

 This ridge is marked in its course by a shoaling of the 

 water at one point, and by two small islands. These islands 

 are within the 200 ft. and 300 ft. contours, and are 29 feet 

 and 66 feet in height, respectively. The nearer of the two 

 is composed of tufaceous material, and the outer one is 

 probably of the same, though it was not seen near at hand. 



The bed of the island is not homogeneous but is composed 

 of numerous minor beds of varying thickness and color, 

 now of the white shale and now of reddish, yellowish, or 

 dark grayish tufaceous material. The thickness of a given 

 bed is seldom the same for any distance, and it frequently 

 happens that a bed will wedge out and disappear within a 

 few rods. One of the beds noted changes in thickness 

 from about six feet to one foot in a little more than fifty feet. 

 The various beds of the formation therefore occur in no 

 fixed order, though the bulk of the deposits is always of the 

 diatomaceous earth. 



