CHAP. Yi.] GEOGKAPHICAL AND GEOLOGICAL CHANGES. 89 



Lyell well remarks that the pure calcareous mud produced by 

 the decomposition of the shelly coverings of mollusca and 

 zoophytes would be much lighter than argillaceous or arena- 

 ceous mud, and being thus transported to greater distances 

 would be completely separated from all impurities. 



Now the Globigerinse have been shown by the ChallengeT 

 explorations to abound in all moderately warm seas; living 

 both at the surface, at various depths in the water, and at the 

 bottom. It was long thought that they were surface-dwellers 

 only, and that their dead tests sank to the bottom, producing 

 the Globigerina-ooze in those areas where other deposits 

 were absent or scanty. But the examination of the whole 

 of the dredgings and surface- gatherings of the Challenger by 



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

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

 proportions : — 



Carbonate of Lime 43*93 to 7947 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 Glohigerina-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 debris 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-oozG of the deep ocean bed. 



