careous Foraminifera in the chalk, and of siliceous Polycystina and vitreous Sponges in the Atlantic mud, may, however, render this rather a question of proportion than of radical difference. I would point out that the "White Chalk (Terrain Senonien) of Touraine varies in colour from white to light yellow, or greyish yellow, is a much less pure carbonate of lime, and is wonderfully rich in siliceous sponges. In fact there is one portion of it, from 28 to 30 ft. thick, which contains no carbonate of lime at all*. At other places in France, and in Europe, the chemical composition of the chalk differs considerably, and the colour varies from white to dark grey.
Mr. Lonsdale† , many years since, pointed out that white chalk was composed largely of microscopic organic debris, consisting chiefly of minute Foraminifera; and Dr. Mantell‡ afterwards estimated that more than a million of such remains are contained in a cubic inch of some of our chalk. I would further draw attention to a remark by Dr. Mantell in the same work (p. 315). Speaking of the chalk,
chalk, as ordinarily understood by geologists or chemists. In order to make a correct comparison of their composition with that of chalk, I was obliged to make analyses of the latter rock, two of which I annex.
Carbonate of lime
Carbonate of magnesia
Insoluble rock debris
Phosphoric acid
Alumina and loss in analysis
Chloride of sodium
Water
Grey Chalk (base of), Folkstone.
94.09 0.31 3.61 traces " 1.29 0.70
100.00
White Chalk, Shoreham (Sussex).
98.40 0.08 1.10 0.42 ... ... 100.00
On the other hand, the specimens of Atlantic mud received from Mr. Gwyn Jeffreys, Dr. Carpenter, and others (about eight in number), and examined by me, contain at highest not 60 per cent, of carbonate of lime, along with very much siliceous and aluminous matters, oxide of iron, &c. ; and if we were even to subtract the amount of water, organic matter, and marine salts found by analysis in them, as these substances would be in greater part removed before such mud could, in the process of ages, be converted into solid rock, the amount of carbonate of lime would be still far less than that present in what would ordinarily be regarded as chalk ; in fact the resulting rock would have the exact composition of many of the older marls or impure limestones."
- Mem. Soc. Geol. de France, ser. 1, vol. ii. p. 239.
† Lyell, Anniv. Address Geol. Soc. for 1836, p. 13.
‡ Wonders of Geology, 6th edit. 1848, vol. 1. p. 305.
