HISTORY OP THE CEETAOEOUS EUNTS. 83 



sample," Mr. Porbes observes in his Eeport, " shows its chemical 

 composition to be as follows : — 



" Carbonate of lime 50*12 



Alumina (with phosphoric acid) soluble in acids 1-33 



Sesquioxide of iron, soluble in acids 2*17 



Silica in a soluble condition 5*04 



Pine insoluble gritty sand (rock debris) .... 26*77 



Water 2*90 



Organic matter 4*19 



Chloride of sodium and other soluble salts . . 7*48 



100*00 



" If we compare the chemical composition as above with that of 

 ordinary chalk, which consists all but entirely of carbonate of lime, 

 and seldom contains more than from 2 to 4 per cent, of foreign 

 matter (clay, silica, &c.), it will be seen that it differs chiefly in 

 containing so very large an amount of rock matter in a fine state of 

 division. If we subtract the water, organic matter, and marine 

 salts, which would probably in greatest part be removed before such 

 mud could in process of ages be converted into sohd rock, even then 

 the amount of carbonate of lime or pure chalk would not be more 

 than, at highest, some 60 per cent, of the mass. As regards the pro- 

 bable origin of the pebbles and gravel found in the various dredgings, 

 it will be seen from the description* that they consist principally of 

 fragments of volcanic rocks and crystalline schists. The former have 

 in all prohahility come from Iceland or Jan Meyen, whilst the latter 

 have probably proceeded from the north-west coast of Ireland." 



The next analysis is by Mr. W. J. Ward, and was made in the 

 Chemical Laboratory of the Geological Museum, Jermyn Street. It 

 is taken from ' The Memoirs of the Geological Survey,' vol. iv. p. 15 

 (by Mr. Whitaker, B.A.), the material being a sample of " the Upper 

 Chalk with Flints " from the vicinity of Gravesend. In this there 

 were in 100 parts of the chalk : — 



Carbonate of lime 98*52 



Ignited insoluble residue, chiefly silica . . '65 

 Sulphate of lime '14 



99*31 



The remaining fractional parts consist of other materials in very 

 minute quantities. 



Lastly, there are two analyses of grey and white Chalk which I 

 quote from an address delivered by the President, Mr. Prestwich, at 

 the Anniversary Meeting of the Geological Society in Peb. 1871 — 

 an address replete with valuable information of aU kinds bearing 



* A separate description is given of these G17 subangular fragments, in gene- 

 ral not above :i to ^ grain in weight, the biggest only weighing 3 grains (' Depths 

 of the Sea,' 1872, Appendix 0, p. 514). 



o 2 



