88 



ISLAND LIFE. 



[part I. 



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 approxi- 

 mately with chalk in chemical composition, 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 carbonate of lime, and a very minute 

 quantity of alumina and silica. This large proportion of car- 

 bonate of lime implies some other source of this mineral, and 

 it is probably to be found in the excessively fine mud produced 

 by the decomposition 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 



1 Sir W. Thomson, Voyage of Cliallenger, Vol. II., p. 374. 



2 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 



Water and loss 1-148 



This chalk consists simply of comminuted corals and shells of tlie reef. 

 It has been examined microscopically and found to be destitute of the 

 minute organisms abounding in the chalk of England. {Geology of the 

 United States Exploring Expedition, p. 150.) 



This absence of Glohigerince is a local phenomenon. They are quite 

 absent in the Arafura Sea, and no Globigerina-ooze was found in any of 

 the enclosed seas of the Pacific, but with these exceptions the Glohigerince 

 " are really found all over the bottom of the ocean." (Murray on Oceanic 

 Deposits— Proceedings of Royal Society, Vol. XXIV. , p. 523.) 



The above analysis shows a far closer resemblance to chalk than that 

 of the Globigerina-ooze of the Atlantic, four specimens of which given by 



