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



valleys (belonging to the present drainage system) is found, 

 in general, below a depth of 250 feet. 



The recent tilting of the island, which has been mentioned, 

 appears to have occurred largely if not wholly during the 

 island's emergence after its reduction to a peneplain. For 

 the constancy of the depth of the more recent submarine 

 features clearly shows that their relative attitude cannot have 

 been appreciably altered since the time of their formation, 

 and therefore that the tilting must have preceded this in 

 greater part, at least. To this recent differential elevation 

 is due, in part at least, the long, narrow channels of the 

 southern side of the island, as contrasted with the open valleys 

 on the north; though these are doubtless due in part, also, 

 to the more rapid cliff cutting on the southern coast. 

 . The present drainage system of the island was begun at 

 the time of the last rise, after the formation of the pene- 

 plain. This peneplain has since been deeply dissected and 

 eroded, till only the roughly levelled summits of the ridges 

 remain to mark its former existence. Sufficient time has 

 elapsed since the streams began their work for the gorge of 

 Silver Canon to be cut down through more than 1,400 feet 

 of rock, while in the same time the broader valley back of 

 Avalon has been excavated and its slopes minutely carved. 

 The topography, then, is by no means young, but it has not 

 passed its prime. The submergence and rapid cliff reces- 

 sion tend to preserve the youthful appearance of the island, 

 by shortening the stream channels, thus increasing their 

 grade and causing the streams to continue their sharp, 

 incisive cutting. To such a cause is due the dissection of 

 the alluvial fan back of Avalon. 



The isthmus is a particularly interesting feature of the 

 island, for the mass is nearly separated at this point. A 

 very slight further subsidence would be sufficient to form 

 two islands. That the isthmus once formed a watershed, 

 which separated the two stream valleys to the north and 

 south, there can be no doubt. These drowned valleys now 

 form the harbors on either side, and constitute the most 

 pronounced example of valley drowning on the island. The 



