ANNIVEKSART ADDEESS OP THE PRESIDENT. xHx 



careous Foraminifera in tlie 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 Senonieu) 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*. A.t other places in 

 Prance, and in Europe, the chemical composition of the chalk 

 differs considerably, and the colour varies from white to dark grey. 



Mr. Lonsdale f, many years since, pointed out that white chalk 

 was composed largely of microscopic organic debris, consisting chiefly 

 of mimite Foraminifera; and Dr. MantelliJ: 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. 



Grey Chalk White Chalk, 



(base of), Shoreham 



Folkstone. (Sussex). 



Carbonate of lime 94-09 98-40 



Carbonate of magnesia 0-31 008 



Insoluble rock debris 3-61 1-10 



Phosphoric acid traces-i 



Alumina and loss in analysis „ J 



Chloride of sodium 1-29 



Water 070 



100-00 10000 



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. ; .snd 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 or- 

 dinarily 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. G^ol. de France, ser. 1, vol. ii. p. 239. 



t Lyell, Anniv. Address Geol. Soc. for 1836, p. 13. 



X Wonders of (Geology, 6th edit. 1848, vol. 1. p. 305. 



