8o THE BUILDING OF AN ISLAND. 



happened in beds laid down in fresh water ponds, where fresh water shells along 

 with the beds themselves have been similarly converted; and associated with 

 them a great quantity of fragments of various species of wood have been so 

 perfectly preserved in the form of agate that the minutest details of their 

 structure are beautifullv exhibited. 



The amount of silica in solution during the deposition both of the strati- 

 fied clays and the later limestones, must have been enormous, for besides the 

 silicified beds and fossils, both marine and fresh water, already mentioned as 

 lying among the clay beds, the present writer has seen in the limestones of the 

 Northwest true flints resembling those found in the chalk, and has seen on the 

 north side of the islet known as Long Island a beach of rolled flint pebbles, 

 very much like such beaches on the south coast of England. 



It appears, then, that while the islands of Anguilla, St. Martin's and St. 

 Bartholomew are possibly to be regarded as geologically belonging to the same 

 axis as our own island, Antigua and the more southerly non-volcanic islands, 

 'including Barbados, are related to the other great axis, namely, that of the vol- 

 canic chain. 



When we now turn westward and follow the chain of the larger islands, 

 .we find that all of them, from Porto Rico to Cuba and to Jamaica, have ex- 

 tensive limestone formations on their seaward slopes and even high up in 

 the mountains, while the central and highest parts are formed of rocks some- 

 what similar, and in some cases very similar, to the older rocks of our own 

 island. These older rocks in the several islands have also been commonly 

 pushed up at high angles, are much altered and contorted, and are penetrated 

 by igneous rocks just as the older rocks of St. Croix. From all which it may 

 be concluded that the principal events in the geological history of St. Croi.x 

 have been common to the whole line of the larger islands. They too have 

 been pushed up, have been denuded, have sunk again and have received the 

 limestone beds on the worn edges of their strata, and then have once more 

 been forced up above the level of the sea. 



Hence we see that the east and west axis of the West Indies is a great 

 mountain chain, which has probably been raised as a whole by the same forces 

 acting through its whole length. We can already see, however, in the differ- 

 ent heights of the various parts, evidence that the conditions have not been 

 alike in all of them ; and a detailed studv of the islands would no doubt reveal 

 many important differences. 



Investigations concerning the distribution of the plants and land shells of 

 the West Indies have shown the great probability of the islands and the banks 

 from which they rise having at one time been continuous land, and having 

 since sunk so as to allow the water to flow over some of the cross valleys and 

 thus to break up continuous chains of mountains into chains of islands. The 

 depths in manv of the channels between the islands exceed a thousand fathoms 

 or six thousand feet. When, however, we recall what we have learned from 

 the studv of the St. Croix rocks, about cross elevations and depressions, it 



