90 



ISLAND LIFE 



PAET I 



the Atlantic basin is very characteristic, and yields an 

 assemblage of forms which represent in a remarkable 

 degree the corresponding group in the white chalk. 

 Species of the genus Cidaris are numerous ; some remark- 

 able flexible forms of the Diademidse seem to approach 

 Echinothuria " ^ Now, as some explanation of the origin 

 of chalk had long been desired by geologists, it is not 

 surprising that the amount of resemblance shown to exist 

 between it and some kinds of oceanic mud should have 

 been at once seized upon, and the conclusion arrived at 

 that chalk is a deep-sea oceanic formation exactly analogous 

 to that which has been shown to cover large areas of the 

 Atlantic, Pacific and Southern oceans. 



But there are several objections to this view which seem 

 fatal to its acceptance. In the first place, no specimens of 

 Globigerina-ooze from the deep ocean-bed yet examined 

 agree even approximately with chalk in chemical compo- 

 sition, only containing from 44 to 79 per cent, of carbonate 

 of lime, with from 5 to 11 per cent of silica, and from 8 to 

 33 per cent, of alumina and oxide of iron.^ Chalk, on the 

 other hand, contains usually from 94 to 99 per cent, of car- 

 bonate of lime, and a very minute quantity of alumina and 

 silica. This large proportion of carbonate of lime implies 

 some other source of this mineral, and it is probably to be 

 found in the excessively fine mud produced by the decom- 

 position and denudation of coral reefs. Mr. Dana, the 

 geologist of the United States Exploring Expedition, found 

 in the elevated coral reef of Oahu, one of the Sandwich 

 Islands, a deposit closely resembling chalk in colour, 

 texture, &c. ; while in several growing reefs a similar 

 formation of modern chalk undistinguishable from the 

 ancient, was observed.^ Sir Charles Lyell well remarks 



1 Nature, Vol. XL, p. 297. 



2 Sir W. Thomson, Voyage of Challenger, Vol. II., p. 374. 

 ^ The following is the analysis of the chalk at Oahu : — 



Carbonate of Lime 92-800 per cent. 



Carbonate of Magnesia 2*385 " 



Alumina 0*250 



Oxide of Iron 0*543 



Silica 0-750 ^'^ 



Phosphoric Acid and Fluorine 2*113 " 



Wa,ter and, los^ 1*143 



