ORIGIN OF ISLANDS. 189 



i; 



CHAPTER XXII. 



ORIGIN OP ISLANDS, — ISLANDS OP CONTINENTAL ORIGIN. — ROCKS OP THE SHORES. — 

 ISLANDS OP DEPRESSION, ELEVATION, AND EROSION.— ISLANDS OP OCEANIC 

 ORIGIN. — ATOLLS AND VOLCANOS. 



On viewing the great geological labours accomplished by the dash 

 of the waves on the various coast-lines, savants have often asked what 

 is the share the sea takes in the formation of islands. Among 

 the lands which are scattered over the surface of the ocean, some 

 disposed in groups or series, and others completely solitary, how should 

 we distinguish between those which the sea has detached from the 

 continents, and those which have existed in an isolated manner from 

 all times like separate worlds ? Is it even possible in the present state 

 of science to attempt a classification of islands, according to their 

 origin ? Yes, this work may be commenced. By calling to our aid 

 the new resources which botany and zoology offer to physical geo- 

 graphy, we may affirm that, sooner or later, we can indicate with cer- 

 tainty the manner of formation, and the relative age of each oceanic 

 country.* 



Firstly, it is evident that the islands, islets, and rocky ledges situ- 

 ated in the immediate neighbourhood of the coasts, are a natural de- 

 pendancy of the continents, and geologically make a part of them. 

 At the base of the high mountains, which send far into the sea ad- 

 vanced capes, similar to the roots of an oak, we may see in many 

 places, so to say, the crests of the lateral chains continue under the 

 surface of the ocean. The outline of the continental heights sinks by 

 degrees ; to the mountains succeed the hills, and the promontory of 

 rocks, whose escarpments plunge beneath the even surface of the 

 waters. An inconsiderable strait, simply a hollow where the waves 

 meet each other, separates the cape from a less elevated island. But 

 further on there opens a wide channel, and the peak which shows itself 

 at the surface on the other side of the submarine valley is no longer 

 anything more than a needle of rock. Beyond stretches the open 



* See especially the works of Darwin, Wallace, and a study by M. Oscar Peschel, der Ur- 

 sprung der Inseln ; Atisland, Jan. and Feb., 1867. 



