| 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 consisting 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, Folkestone. | White Chalk, 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 débris | 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 débris, at once distinguish true chalk from the Globigerina-ooze of the deep ocean bed.
[24] Notes on Reticularian Rhizopoda; in Microscopical Journal, Vol. XIX., New Series, p. 84.
[25] Proceedings of the Royal Society, Vol. XXIV. p. 532.
[26] See Presidential Address in Sect. D. of British Association at Plymouth, 1877.
[27] Geological Magazine, 1871, p. 426.