60 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



and honeycombed, and eaten away at the base. On the gulf side of the 

 Beminis, and all the way north to Great Isaac, we meet tine patches of 

 corals in from four to six fathoms of water. These patches and bars 

 extend but little way eastward on the bank, and flourish most luxuri- 

 antly as we approach the Gulf Stream edge of the bank out to fifteen 

 or sixteen fathoms, where they often cover extensive tracts separated 

 by irregular sandy lanes. 



Great Isaac. 



Plates XXIV. to XXVI. 



Great Isaac, as it is approached from the south (Plate XXV.), reminds 

 one of Double Headed Shot Cay, with its rounded outline and outlying 

 islets and rocks presenting a most barren and desolate appearance. The 

 southern faces of the island, and of its adjoining islets and rocks, are all 

 cut off by low vertical cliffs, full of caverns and cavities. The surface 

 of Great Isaac is an admirable example, perhaps one of the best, of the 

 forces which have been at work during the subsidence of the Bahamas. 

 The aeolian structure of the rocks is everywhere most apparent. The 

 action of the sea in undermining the shore slopes and forming low ver- 

 tical cliffs by the breaking off and falling into the sea of huge angular 

 masses of rocks is seen on all sides. The action of the sea during hur- 

 ricanes in throwing up huge masses far above high-water mark is not 

 better shown on any other island of the group. Finally, the action of 

 the salt-water spray and waves, as well as of the tropical rains, in pit- 

 ting and honeycombing the surface of the island, the former near the 

 shores, the latter everywhere else over the cay, cannot fail to strike the 

 eye of the most casual observer. As we passed the west end of Great 

 Isaac we observed a number of huge isolated irregular rocks, left far 

 above the high-water mark, passing into an irregular mass of angular 

 blocks forming a rude broad wall, left as a witness of the fury of the 

 hurricane of 1870. ]iut on the north face of the island we come upon 

 another tract covered by still larger blocks, thrown up by an older hur- 

 ricane, the date of which is not known. Two of the largest of these 

 blocks we measured, and found them to be, the one twelve feet eight 

 inclies long by four feet wide and six feet high, at a distance of one 

 hundred and twenty-five feet from the shore and fully twenty-five feet 

 above high-water mark ; the other was fifteen feet six inches long eleven 

 feet wide and six feet liigh, at least eighty feet from the shore and about 

 eighteen feet above high-water mark (Plate XXIV.). On the highest 



