190 A NATURALIST ON DESERT ISLANDS. 



quite possible that at another season of the year they may 

 visit the group to breed.* 



Situated on the extreme northernmost edge of the 

 shallow submarine shelf, which extends a varying distance 

 from the mainland, "The Seven Brothers" form, 

 so to speak, another link in the chain of islands standing 

 Hke outposts along the frontier of a great continent. 



Immediately to their north, the submarine plateau 

 from which they rise sinks deeply and suddenly down 

 to a depth of five hundred fathoms, and from that, almost 

 precipitously, and in giant steps, to depths of 1,000, 

 1,500 and 2,000 fathoms, until in little more than twenty 

 miles in a north-westerly direction, the abysmal depth 

 of over 2,500 fathoms is attained in the marine depression 

 which goes by the name of the Puerto Rico Basin (see 

 map). Consequently, if we could imagine this basin 

 drained of all water, and ourselves standing at the bottom 

 of it, looking southwards, we should see before us a long 

 barrier chain of mountains rising steeply to a height of 

 15,000 feet, and in one culminating spot, seven little 

 isolated peaks rising in a group to another five or six 

 hundred feet. There is, in fact, reason to suppose that 

 in a past geological age one could have done this in 

 actuality, as we try to do it now in imagination ; but this 

 was when the land had probably risen on such a vast 

 scale that the Caribbean basin was very nearly a dry plain 

 sloping to the west.^j* 



Now, like buoys which mark that something has been 

 sunk below, these islets, mere relics of ancient land, shew 

 their heads above the water. Like sentinels, they stand in 

 lonely isolation, outposts of a great land, exposed to the 

 full force of the Trade-winds and the fretful Caribbean 



♦ Since our visit, Mr. Ferry on behalf of the Field Musetun of Natural 

 History, Chicago, found both the Sooty Tern and the Noddy Tern 

 breeding on the island in February, 1909, ( ** Birds of the Leeward 

 Islands, Caribbean Sea," Field Mus. Nat. Hist. No. 137, by Chas. B. Cory). 



t Spencer, ♦* Beconstruction of the AntilUan Continent," Bull, 

 Geol. Soc. Amer., vol. vi., pp. 103-140. 



