1882.] 129 [Crosby. 



scoured out a large part of the sand and gravel which it had 

 deposited and cut a narrow channel through the reef itself. 

 Daring this period of elevation, Cuba, like most rapidly rising 

 lands, had few harbors ; but when subsidence began, the sea 

 occupied the channels and basins which had been excavated and 

 cleared out by the rivers, and thus a large number of harbors 

 came into existence. 



Opposite the mouths of the larger rivers, such as the Toar and 

 Molasses in the vicinity of Baracoa, the reef in question was com- 

 pletely interrupted, and these streams discharge into broad, open 

 bays ; while the lower portions of their valleys show, equally with 

 the harbors, that the land is sinking. They are half drowned 

 valleys filled to a considerable depth with land detritus, condi- 

 tions which could not exist if the land were rising or had recently 

 risen. 



But the most satisfactory evidence that the ancient reefs of 

 Cuba were not formed during periods of elevation is found in 

 the great thickness of the reefs themselves. The reef which, in 

 eastern Cuba, reaches a height of five hundred feet above the sea 

 includes not less than four hundred feet in vertical thickness of 

 coral-rock, and in this estimate no allowance is made for what 

 the reef has lost by erosion. The giant reef of which El Yunque 

 is, perhaps, fhe last remnant, has still, after suffering enormous 

 waste, a^ thickness of more than one thousand feet. While, ac- 

 cording to Mr. Sawkins, the maximum thickness of the ancient 

 reefs of Jamaica is two thousand feet. It follows, from Mr. 

 Agassiz's theory, that the El Yunque reef, for example, began to 

 grow in water much more than one thousand feet deep ; for to 

 the present thickness of the reef we must add, not only what it 

 has lost by erosion, but also the amount of elevation which took 

 place during the ages when the reef was growing. A depth of 

 even one thousand feet is, however, entirely inadmissible, in Adew 

 of the well established fact that reef-building corals are limited to 

 depths of less than twenty fathoms. 



We have, then, apparently, no resource but to accept Darwin's 

 theory as an adequate explanation of the elevated reefs of the 

 Greater Antilles ; and, therefore, to admit that the upheaval of 

 this portion of the earth's crust has been interrupted by periods of 

 profound subsidence during which the reefs were formed. 



PROCEEDINGS B. S. N. H. VOL. XXII. 9 JUNE, 1883. 



