CHAP. VI GEOGRAPHICAL AND GEOLOGICAL CHANGES 91 



that the pure calcareous mud produced by the decompo- 

 sition of the shelly coverings of mollusca and zoophytes 

 would be much lighter than argillaceous or arenaceous mud, 

 and being thus transported to greater distances would be 

 completely separated from all impurities. 



Now the Globigerina3 have been shown by the Challenger 

 explorations to abound in all moderately warm seas ; living 



This chalk consists simply of comminuted corals and shells of the 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.) Mr. Guppy also found 

 chalk-like coral limestones containing 95 p.c. of carbonate of lime in the 

 Solomon Islands. 



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

 absent in the Arafura Sea, and no Globigcrina-oozQ 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 Glohigerina-oozQ of the Atlantic, four specimens of which given by 

 Sir W. Thomson {Voyage of the Challenger Vol. II. Appendix, pp. 374- 

 376, Nos. 9, 10, 11 and 12) from the mid- Atlantic, show the following 

 proportions : — 



Carbonate of Lime 43-93 to 79 "17 per cent. 



Carbonate of Magnesia 1*40 to 2-58 



Alumina and Oxide of Iron. 6 '00? to 32-98 



Silica 4-60 to 11-23 " 



In addition to the above there is a quantity of insoluble residue consist- 

 ing of small particles of sanidine, augite, hornblende, and magnetite, 

 supposed to be the product of volcanic dust or ashes carried either in the 

 air or by ocean currents. This volcanic matter amounts to from 4*60 

 to 8 '33 per cent, of the Globigerina-ooze of the mid- Atlantic, where it 

 seems to be always present ; and the small proportion of similar matter 

 in true chalk is another proof that its origin is different, and that it was 

 deposited far more rapidly than the oceanic ooze. 



The following analysis of chalk by Mr. D. Forbes will show the difference 

 between the two formations : — 



Grey Chalk, White Chalk, 

 Folkestone. Shoreham. 



Carbonate of Lime 94*09 98*40 



Carbonate of Magnesia 0'31 0-08 



Alumina and Phosphoric Acid .. a trace 0*42 



Chloride of Sodium 1 -29 — 



Insoluble t^^&r?:^ 3-61 1*10 



(From Quarterly Journal of the Geological Society, Vol. XXVII.) 



The large proportion of carbonate of lime, and the very small quantity 

 of silica, alumina, and insoluble debris, at once distinguish true chalk from 

 the Globigerina-ooze of the deep ocean bed. 



