220 CORALS AND CORAL ISLANDS. 



The rock of the surface is described as a calcareous sand- 

 rock, analogous evidently to the beach sand-rock and drift 

 sand-rock. Toward the shores the solid reef-rock outcrops 

 — a hard, white limestone. Lieut. Nelson speaks of that on 

 St. George's Island, as a " very hard, fine-grained or compact 

 limestone, in which scarcely a vestige of organic structure is 

 to be seen. 1 ' In one place he observed a Masandrina (Di- 

 ploria) four feet above high-tide level. 



The soil is calcareous, modified by vegetation and in part, 

 according to Lieut. Nelson, " a dry, aluminous earth." The 

 same observer mentions the occurrence on the land of oxide 

 of iron and manganese, and of some titanic iron ; but Mr. J. 

 Matthew Jones states (Canadian Naturalist, Feb. 1864) that 

 all stones not of coral and shell origin have undoubtedly 

 been brought in the roots of drift-trees ; and the West Indies 

 were probably their source. 



The greater part of the old atoll is still a submerged reef. 

 Its outer border is mostly from one to three fathoms under 

 Avater at low tide, though in some parts laid bare at the ebb. 

 It has open channels at a (called the Chub cat), b (Blue cut, 

 shallow), c (N. W. Channel), d (N. E. Channel), e (Mills' 

 Breaker Channel), f (The Channels affording the nearest 

 routes to Murray Anchorage and St. George's Harbor), g 

 (Channel by St. David's Head, shallow), and h (Hog-fish cut). 

 The reef-grounds, inside, are encumbered with countless clumps 

 of corals and coral -heads, one to four fathoms under water with 

 intervals between of five to ten fathoms ; some large tracts are 

 without corals, and these have a nearly uniform depth of seven 

 or eight fathoms. To a vessel entering, the positions of the coral 

 clumps are made known by the brownish or discolored water 

 above them. The bottom, over large areas, is a calcareous clay 

 or mud ; that of Murray Anchorage, a fine chalky clay. 



