150 CORAL AND ATOLLS 



appearance. The " brown mud " is made up mostly of the 

 siliceous remains of Badiolaria, &c., mixed freely with a 

 brownish unorganised deposit. 



From al] samples there was a fairly large proportion of 

 this unorganised debris, which consisted, in great part, of 

 particles of pumice and irregular mineral crystalline forms, 

 and with the deposit, from greater depths, a large admixture 

 of the spicules of sponges was always found. From the limit 

 of the drop of the reef, therefore, the foundation of the atoll 

 would seem to be a great sedimentation slope, and certainly 

 not a steep peak, rising suddenly from ocean depths to a wave- 

 washed reef. The slope is certainly nothing like the picture 

 that Guppy — its last investigator — made of it, for he said in 

 a letter to Sir John Murray, published in Nature, p. 237, 

 1889 : " Here the submarine slope slopes gradually down to 

 20 or 30 fathoms, but beyond this the descent is precipitous." 

 The steep descent comes first, the gradual slope comes after. 

 We may take the slope from the reef drop, to 2000 fathoms 

 — that is to a distance of ten miles from the atoll — at an 

 almost uniform gradient of one in five. From ten to fifty 

 miles the slope is about one in eighty, and from that point 

 onwards, at about one in a hundred, to the depths of the 

 ocean. In other words, the Gloligerma ooze slope is one in 

 five ; the Badiolaria ooze slope is about one in a hundred. 

 When a section of this great ridge of ocean floor, with its flat 

 top of reef, is made to scale, it is at once seen that a very 

 wrong importance has been ascribed to certain features of an 

 atoll. The islands are trivial things, the lagoon merely a 

 slightly submerged reef, and the barrier an inconsiderable 

 ring around its edge ; it is only by a proper understanding of 

 the relative proportions of these parts that a right interpreta- 

 tion of their origin may be arrived at. 



