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



are broad and open stream valleys, while those running 

 down to the opposite shore have a long and trough-like 

 character which they preserve to the shore-line. These 

 two types are quite pronounced over all the southeastern 

 division of the island, while at the isthmus the two harbors 

 which are merely submerged stream valleys still show 

 the same contrast. In the western end this characteristic, 

 though still evident, is not so marked. The narrow, trough- 

 like canons are occasionally somewhat broader in their upper 

 portions than near their mouths, where they are frequently 

 mere rocky gorges. The most pronounced example of the 

 narrow type is Silver Canon, whose walls near the mouth 

 rise to a height of over 1,000 feet, while the distance between 

 them at the base is in places not more than from twenty-five 

 to a hundred feet. The length of this canon is about three 

 miles. Avalon Canon, a good example of the other type, 

 has a length of about two miles, with a mean width, from 

 watershed to watershed, of somewhat more than that. 

 From the main ridge on either side a great number of 

 rather short and steep V-shaped canons are tributary to 

 the main valley, these stream beds making the descent of 

 1,200 or 1,400 feet within an average distance of about a 

 mile. 



All the forms of topography thus far described are largely 

 independent of the material from which they are carved; 

 that is, variation in the character of the rocks has but little 

 connection with variation in topographic form. 



2. MINOR FEATURES. 



Echo Lake. There is one small lake on the island, sit- 

 uated about a mile to the northeast of Black Jack, at an 

 altitude of about 1,300 feet. This belongs to the class of 

 ephemeral lakes. Visiting it two summers in succession, at 

 the same season, the writer found it, the first time, a shal- 

 low pond about 100 yards long, while the next year it was 

 entirely dry. It is a small drainage lake, without outlet, 

 probably shut in by faulting. 



