THE BUILDING OF AN ISLAND. 67 



The large blocks may especially attract our notice, but the smaller blocks 

 and those stones that break off from the limestone beds everywhere and cover 

 the surface over a large part of that formation, and the "gravel" of the older 

 formation, are, on account of their great abundance, of much more importance. 

 And in regard to them we notice that they must be continually shifted down 

 the hillsides and farther and farther along the plains, every heavy rain that falls 

 moving some of them, till they also are carried away to the sea. 



The scattered blocks from the limestones are especially abundant on the 

 steep slopes of the hills which form the northeastern edge of the Central Slope, 

 but may be seen everywhere over the surface of the marl district. Sometimes 

 they are corals, but more commonly are only blocks naturally formed by the 

 jointing which we have seen both the limestone and the blue-beach layers to 

 possess. Smaller fragments from the strata fill the soil. In the older forma- 

 tion the pieces broken out of the layers of rock and the finer material result- 

 ing from their decomposition under the action of the weather, make up to- 

 gether a coating of debris over the bottom rock which sometimes' reaches a 

 thickness of from twenty to thirty feet, and effectually conceals the rock be- 

 low. Where this coating is shallow, as it commonly is on the slopes, the rills 

 sometimes cut through it down to the original rock and cut up a portion of 

 that rock to add to the gravelly covering. In the watercc^urses we can see 

 how this covering is brought down by the streams and rearranged to be again 

 carried farther on, till at last, though perhaps after the lapse of centuries, it 

 must eventually reach the sea. 



We have seen, then, that over the whole surface of the island the destruc- 

 tive forces, chemical and mechanical, are constantly at work; but in any case 

 their action in lowering the general level of the country must be extremely 

 slow. 



Perhaps it is next to impossible here in St. Croix, where the streams are 

 so short and so numerous, to come to any clear idea of the rate at which the 

 island is being cut down and carried into the sea, but in larger countries, where 

 the waste from extensive districts, or even great regions, is accumulated in one 

 large river and by it carried into the sea, it is possible to make a rough com- 

 putation of the amount of stuff carried yearlv to the sea, and with the extent 

 of the area known from which this stuff is brought, it is thus possible to say at 

 what rate the area in question, on the average, over the whole surface, is being 

 worn down. Professor Huxley tells us, for example, in his " Physiography," 

 that the Thames carries yearly to the sea fourteen millions of cubic feet of solid 

 matter, and that the effect of this immense work is to lower the land of the 

 Thames basin by the eight-hundredth part of an inch in a year. In other 

 words, it would take 800 vears to lower the level of that tract of country by 

 the average amount of one inch, and nearlv ten thousand years to lower it a 

 foot. Ten thousand years ! And yet the difference in the landscape would 

 not be perceptible, except, perhaps, in some particular spots! 



The rainfall in St Croix is double that of the Thames vallev, but in other 



