AGASSIZ: BAHAMAS. 107 



The action of the water on the bottom of the lagoon is clearly seen in 

 all the patches of clear sand by well defined ripple marks, showing the 

 unstable condition of the sand even at depths of six to seven fathoms. 



Owing to the steepness of the edge of the Hogsty bank, the 100 

 fathom line represents very closely the width of the growing belt of 

 corals. The approximate centre of the inner line of breakers is within 

 one hundred yards of the inner 3 fathom line. I was informed by 

 our pilot that both Northwest and South Cays had increased in size, 

 the former in length on the northerly face, the latter on both its east 

 and west faces. Northwest Cay is mainly built up of the broken frag- 

 ments of corals thrown up on its northern faces derived from coral 

 heads to the eastward and westward of the cay, and South Cay from 

 the heads growing either on the spit to the northwest or to the east- 

 ward on the southern face of the atoll. The cays are nearly flat, with 

 steep shores about fifteen feet high, their surfaces covered with frag- 

 ments of broken shells and corals, and with here and there small patches 

 of vegetation consisting mainly of the different plants so characteristic of 

 the shore line of the Bahama Islands. On parts of the shores of the 

 cays the recent coral rock forming the underlying base of the islets can 

 be seen sloping seawards. The cays at the time of our visit were occu- 

 pied by a rookery of boobies. 



One cannot fail to be struck with the fact that on Hogsty Reef the 

 most luxurious growth of corals is from a depth of five or six fathoms 

 to ten or twelve, and that below that point, while still flourishing, the 

 heads are not clustered as closely together as in shallower water, while 

 the heads similarly diminish rapidly in number as they reach the line of 

 breakers. Contrary to the observations of Bourne x and Hickson, who 

 lay great stress on the growth of corals inside of the lagoons of the atolls 

 they have examined, in our atoll their growth is practically limited to a 

 very narrow belt close to the Hue of breakers, of not more than one 

 hundred to one hundred and fifty yards from the principal line of the 

 corals awash. The slope from the ring of the atoll to about ten or fifteen 

 fathoms is quite moderate, while beyond that it drops very fast. It is 

 on that edge, as has been observed by others, and as is specially marked 



1 Bourne states (Proc. Royal Society, March 22, 1888, p. 440) that corals thrive 

 remarkably and most vigorously in that part of the Diego Garcia Atoll which is 

 most remote from the influence of currents, and he attributes to corals a greater 

 capacity as vegetable feeders than has been supposed. But no decayed vegetable 

 masses derived from the shores of the lagoon are found in either of the atolls I 

 have examined, Alacran or Hogsty. In these atolls there is nothing else for the 

 corals to feed upon except what is brought by the currents of the sea. 



