80 THREE CRUISES OF THE " BLAKE." 



of Mr. Murray, that the inorganic material held in suspension 

 in the sea became precipitated on the sides of volcanic peaks or 

 slopes, and thus forms little by little the base or plateau upon 

 which coral reefs eventually grew. If this be true, it is not 

 necessary to resort to Darwin's theory of extensive areas of ele- 

 vation or of subsidence in order to explain the formation of 

 atolls or of fringing reefs. In the tropics and in regions situ- 

 ated in the path of great oceanic currents which carry along 

 their course an immense amount of pelagic life, serving as food 

 for the animals living upon the bottom, we have all the elements 

 of the gradual accumulation of submerged land, which, when it 

 rises to a certain level, becomes the foundation upon which reefs 

 are formed. In fact, as has been well pointed out by Mr. 

 Murray, we should have in an area of elevation as well as one 

 of subsidence all the elements necessary for the construction of 

 atolls. 



In a very interesting article on the Bermudas, Rein ^ has 

 taken very much the same view of their gradual building up, 

 and explains the present condition of things by causes greatly 

 differing from those adduced by Darwin to account for the 

 apparent atoll shape of the groups. 



The islands composing the Tortugas (Fig. 38) are Logger- 

 head, Bird, Garden, Long, Sand, Middle, and East Keys. These 

 are always above the level of the sea, while Southwest Key and 

 Bush Key are exposed only at low water, and North Key and 

 Northeast Key have disappeared. These insignificant islands 

 are the outcrops of extensive submarine banks. Loggerhead 

 Key, not more than three fourths of a mile in length, is the top 

 of a bank about five miles in length, extending to the three- 

 fathom line, with an average width of three fourths of a mile, 

 and has extensive coral sand flats running in prolongation of 

 the northern and southern extremities of the key. Between this 

 and the Garden Key and Long Key Bank, there are a few 

 shoals running more or less parallel with Loggerhead Bank, the 

 largest of which are Brilliant and White Shoals. Garden Key 

 and Long Key Bank form a rectangular shoal of nearly the 



^ Rein, J. J. Beitrage zur Physika- Bericht. Senckenb. Naturf. Gesell., 1869- 

 lischen Geographic der Bermuda Inseln. 70 (Mai, 1870), pp. 140-158. 



