4 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



worthy that, in spite of the oftentimes luxuriant vegetation, 

 the soil-covering is generally very thin, and the underlying 

 formations are constantly exposed. 



II. TOPOGRAPHY. 



i. MAJOR FEATURES. 



Main Ridge. The island is traversed from end to end by 

 a single main ridge, with branch ridges running out on either 



FIGURE i The Isthmus, looking south. 



side. Beginning about a mile from the southeastern extrem- 

 ity of the island, this ridge makes a bold sweep around the 

 head of Avalon Canon to a point nearly west of Avalon. 

 There it makes an abrupt turn, almost at right angles, and 

 then follows very nearly the line of the northern coast, at an 

 average distance of about a mile from the shore, till it reaches 

 the isthmus. (See fig. i.) This is alow divide, in the form 

 of a saddle, with very gentle slopes. It has a length, 

 between the bounding hills, of less than a quarter of a mile, 



