230 Formation of Sea-Cliffs. paet n. 



the cut-off lava streams very gently inclined, and the 

 bottom of the sea has nearly a similar slope all round 

 the island. How, then, has all the hard basaltic rock, 

 which once extended beneath the surface of the sea, 

 been worn away ? According to Captain Austin, the 

 bottom is uneven and rocky only to that very small 

 distance from the beach within which the depth is from 

 five to six fathoms ; outside this line, to a depth of 

 about 100 fathoms, the bottom is smooth, gently in- 

 clined, and formed of mud and sand; outside the 100 

 fathoms, it plunges suddenly into unfathomable depths, 

 as is so very commonly the case on all coasts where 

 sediment is accumulating. At greater depths than the 

 five or six fathoms, it seems impossible, under existing 

 circumstances, that the sea can both have worn away 

 hard rock, in parts to a thickness of at least 150 feet, 

 and have deposited a smooth bed of fine sediment. 

 Now, if we had any reason to suppose that St. Helena 

 had, during a long period, gone on slowly subsiding, 

 every difficulty would be removed : for looking at the 

 diagram, and imagining a fresh amount of subsidence, 

 we can see that the waves would then act on the coast- 

 cliffs with fresh and unimpaired vigour, whilst the 

 rocky ledge near the beach would be carried down to 

 that depth, at which sand and mud would be deposited 

 on its bare and uneven surface : after the formation 

 near the shore of a new rocky shoal, fresh subsidence 

 would carry it down and allow it to be smoothly 

 covered up. But in the case of the many cliff-bounded 

 islands, for instance in some of the Canary Islands and 

 of Madeira, round which the inclination of the strata 

 shows that the land' once extended far into the depths 

 of the sea, where there is no apparent means of hard 

 rock being worn away — are we to suppose that all these 

 islands have slowlv subsided ? Madeira, I mav remark, 



