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 Kast 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 
1 Rein, J. J. Beiträge zur Physika- Bericht. Senckenb. Naturf. Gesell., 1869- 
lischen Geographie der Bermuda Inseln. 70 (Mai, 1870), pp. 140-158. 
