148 THREE CRUISES OF THE " BLAKE." 



than in a true oceanic area." ^ There is no proof that massive 

 formations are deposited anywhere except along continental 

 shelves, while at the greatest depth we may find the true red 

 clay, the result of chemical decomposition. 



Mr. J. S. Gardner considers the blue m"ud found around shores 

 and in partially closed seas, and passing into a deep-sea deposit 

 at a distance from land, as the equivalent of the gault. The 

 gault has among its mollusca, Leda, Limoj)sis, and Denta- 

 Hum, all well-known deep-sea mollusca. Its foraminifera and 

 echinoderms likewise are deep-water types. The upper gault 

 represents a deeper sea ; blue muds are replaced by green muds; 

 from that we pass to chalk marl, which is apparently a sort of 

 globigerina ooze, and finally to the white chalk, with its great 

 extent and thickness, which proclaim it to be an oceanic or a 

 deep-water deposit ; and there seems to be nothing with which 

 it can be compared except globigerina ooze. 



Professor Liversidge describes a chalk formation from New 

 Zealand, consisting of eighty-one per cent of carbonate of lime 

 and seven per cent of silica. This contains, as is stated by 

 Brady, species of foraminifera now found in deep-sea specimens 

 of globigerina ooze from fifteen hundred to two thousand fath- 

 oms in depth in the South Pacific. The origin of the New 

 Britain chalk is not known, but it is probably recent ; the speci- 

 mens are thrown up on beaches after storms. White chalk con- 

 tains ninety-eight per cent of carbonate of lime, and gray chalk 

 ninety-four per cent. The " Blake " also dredged off Nuevitas, 

 in 994 fathoms, recent chalk which Murray says is more like 

 white chalk than anything he has seen. 



Chalk is certainly not derived from the disintegration of coral 

 reefs, although we frequently find patches of rock associated 

 with coral reefs closely resembling true chalk, both in texture 

 and composition, — as, for instance, in the so-called modern 

 chalk of Oahu. The percentage of carbonate of lime com- 

 posing reef corals ^ varies from ninety-five to ninety-eight ; this 



^ Chalk may be forming at very great ^ Sharpies gives the following per- 



depths close to a continent. See the an- eentages of carbonate of lime : Oeu- 



alysis of the modern chalk collected off Una, 95.37 ; Manicina, 96 54 ; Madrepora, 



Nuevitas in 994 fathoms. (Murray, Bull. 97.19 ; Siderastrea, 97.30 ; Millepora, 



M. C. Z. XII., No. 2.) 97.46 ; Agaricia, 97.73. Madrepora pal- 



