398 PRINCIPLES OF STRATIGRAPHY 



Of 41 slopes determined on the Bahama Islands the lowest is 

 0° and the highest 28° 18'. Seventy-five per cent, of the slopes are 

 below 10°, while one-third of the entire number of determined 

 slopes falls below 5°. 



On Keeling Island, on the other hand, only a few slopes fall be- 

 low 10°. Nearly half the number of slopes recorded by Dietrich 

 lie between 30 and 43 degrees, one being as high as 63° 21'. 



A section made by Darwin on this reef showed that "the water 

 deepens for a space between one and two hundred yards wide, very 

 gradually to 25 fathoms (45.72 meters), beyond which the sides 

 plunge into the unfathomable ocean at an angle of 45°. To the 

 depth of ten or twelve fathoms (18-22 meters) the bottom is ex- 

 ceedingly rugged, and seems formed of great masses of living coral 

 similar to those on the margin." {Da.vwm-21 : 22.) The corals 

 and Hydrozoa growing here were Millcpora alcicornis, Madrepora 

 cf. corymhosa, P-orites and Astrsans. The madrepoYe was not found 

 in the shallow part of the reef nor in the lagoon, and this and the 

 Astr?eans seem to be restricted to the outer slope. Below 12 

 fathoms and especially at a depth greater than 20 fathoms, the bot- 

 tom was covered with coral sand, while at 200 to 300 fathoms, and 

 even at 360 fathoms, the floor consisted of finely triturated frag- 

 ments of stony zoophytes, particles of the lamelliform genera not 

 being recognized, while shell fragments were rare. (Fig. 78.) 



At a distance of 2,200 yards from the breakers, no bottom was 

 found with a line 7,200 feet in length, hence the submarine slope 

 of this coral island is steeper than that of any volcanic cone. (Dar- 

 win.) Submarine clifl^s are also indicated. 



In the shallow part of the reef Porites in places forms nearly 

 the entire floor in great, irregular, rounded masses from four to 

 eight feet broad and little less in thickness. These mounds are sep- 

 arated from each other by narrow, crooked channels about six feet 

 deep, generally intersecting the reef at right angles. On the upper 

 surface of these masses the polyps were often dead, but growth oc- 

 curred around the margin. This was due to the exposure of these 

 corals at lowest tides, the upward limit of growth having been 

 reached. Further seaward the entire mass was alive. The same 

 is true of the Millepora. Closely within the line marked by these 

 partly dead polyps, and thus above the zone of coral growth, flourish 

 three species of nullipores, one thin like a lichen, one with stony 

 knobs as thick as a man's finger, and one moss-like, with thin, rigid 

 branches. These also require the breakers, for they do not exist in 

 any abundance in the protected hollows in the back part of the reef. 

 They thus form only a fringe about 20 yards in width, but located 



