138 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



But perhaps the most characteristic of the Bahama banks, showing 

 the disintegration of the larger seolian islands into small isolated heads, 

 is the Turk's Islands Bank, where the whole area to the south and west 

 of Grand Turk Island, as far as Salt Cay on the one side and East Cay 

 on the other, is one maze of heads with deep channels between them 

 (Plate IX. Figs. 5, 6). 



The "coral heads" described by Mr. J. A. Whipple 1 in his Journal, 

 referred to by Professor Dana 2 as standing in fifty feet near Turk's 

 Island, are not coral heads in the strict sense of the word. They are 

 mushroom-shaped rocks of seolian structure, veneered with coral growth. 

 Similar mushroom-shaped l'ocks of volcanic origin covered with corals 

 occur in Kaneohe Bay, on the weather side of Oahu in the Sandwich 

 Islands, and are common in the Bahamas and Bermudas. 



These tracts of coral heads are seen to be in areas accessible to the 

 prevailing winds, and to the food supply swept upon the banks by them 

 and the prevailing currents. These patches, forming the Middle Ground 

 or " coral heads " region, are without doubt clusters of seolian rock, which 

 have not become entirely disintegrated, and which rise to very varying 

 heights above the bottom of the bank. Upon these in many places 

 corals and Gorgonians have obtained a foothold. Upon the smaller 

 banks, closed on one or more sides, the corals or coral heads extend 

 only a short distance upon the side open to the action of the prevailing 

 winds. 



Upon Little Bahama Bank we find no extensive tract of coral heads 

 outside of those areas which may be called edging or fringing reefs, or, 

 as on the southern face of Bahama Island, their extension a short dis- 

 tance upon the bank. The edging reefs may in general be said to exist 

 along the margin of all the Bahama banks in from three to ten, or even 

 fifteen to eighteen fathoms of water. 



There seems to be nothing in the distribution of the coral reefs upon 

 the Bahamas to show that they form anything more than comparatively 

 thin layers of coi-als overlying the asolian rocks which constitute the 

 substructure of the islands and the smaller sunken patches, and the foun- 

 dation of the banks. What their thickness or extent must have been in 

 order to form the extensive beaches which supplied the material which 

 was blown into the dunes of the Bahama seolian land, is another ques- 



1 " The trunk which made up two thirds of its height was only fifteen feet in 

 diameter along its upper half, and it supported above a great tabular mass one 

 hundred feet in diameter, whose top was bare at low tide." 



2 Corals and Coral Islands, p. 133. 



