68 THE BUILDING OF AN ISLAND. 



respects the destructive agencies are probably not greater, and if we allow 

 the results here to be twice as great as what is known in the Thames valley we 

 see that it would take 400 years to lower the surface of our island on the aver- 

 age by a single inch, and a similar statement, varying according to conditions, 

 may, of course, be made in regard to other lands. No wonder then that we 

 speak of the '' everlasting hills'' and regard everything as permanent. Yet it 

 is certain that there have been enormous destructions of the land masses hard 

 masses, too, in such a way that they have been ground down, not by a single 

 foot, but by hundreds, or even thousands, of feet, so that we can only conceive 

 that geological time must be reckoned rather by millions than by thousands of 

 years. What an immense period we must allow, for example, for the cutting 

 down of those enormous folds of tbe hard rocks of our older formation, which 

 we have seen must have formerly existed, but which have been planed down 

 to a small fraction of what they must once have been ! 



Such calculations as the above have, however, only a general value, for we 

 have no means of judging of the rainfall of the past,* neither is it possible to 

 state, except in a wide fashion, under what other conditions the destructive 

 forces acted ; we must, therefore, remain content with the general conclusion 

 which the study of the strata and the study of the present conditions force 

 upon us, namely, that vast periods of time must be allowed for the sculpturing 

 of the surface of our island into the forms which now adorn it. And this looks 

 no farther back than to the last elevation, when the mass of the "blue-beach" 

 rocks was thrust up bearing the limestone beds on its surface. And here we 

 are brought to a question which will considerably affect our conception of the 

 work which has been done and the time which has been taken in this last great 

 and still continuing process of the sculpturing of our island's surface. How 

 far did those limestone strata then lifted up extend over that surface ? Did 

 they extend over the whole of it, so that we must regard the blue-beach rocks 

 as onlv exposed to our view by the partial removal of the limestones? That 

 may have, indeed, been so. It may be that the whole of the east end and 

 north side of the island were covered with the limestone beds as well as the 

 centre and south which are now so covered ; but we have, of course, no right 



It should be remembered that the rainfall depends largely on the elevation of the land, and may 

 vary greatly among islands situated in the same geographical region and even when they are near to each 

 other. Porto Rico is not more than about 70 miles distant from Santa Cruz and very nearly in the same 

 latitude ; but it has elevations three times as high as ours, and in a large part of its mountain region the 

 rainfall is more than double ours. 



In connection with this subject it may be noted that the July number of the Monthly Weather 

 Review for 1906 contains an instructive article from the pen of Mr. William H. Alexander of the Meteor- 

 ogical Station at San Juan, on the meteorology of Porto Rico, in which he gives a map showing the distri- 

 bution of the annual average rainfall in the island. Only in one part, lying along the south coast of 

 Ponce and its neighbourhood, is the rainfall so mTOiy as in St. Croi.x, namely, under fifty inches (ours is 

 about forty-seven inches) ; the fall is rather larger (fifty to seventy inches) in the e.xtrerae northwest, then 

 towards the interior, seventy to ninety inches ; still higher, ninety to one hundred and ten inches ; and 

 around El Yunque, the highest mountain (about 3,300 feet) the fall is over one hundred and ten inches. As 

 a consecjuence of the great rainfall, streams which are considerable in the rainy season run to the coa.st, 

 cutting out deep ravines and leaving sharp ridges between them. It follows that the work of levelling 

 must go on in Porto Rico at a much faster rate than with us. 



