chap, vi.] GEOGRAPHICAL 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 Globigerinsc have been shown by the Challenger 
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. 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 : — 
Carbonate of Lime... 
Grey Chalk, 
Folkestone. 
... 94-09 
White Chalk, 
Shoreham. 
98-40 
Carbonate of Magnesia 
... 0-31 
0 08 
Alumina and Phosphoric Acid..., 
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-ooze of the deep ocean bed. 
