84 DR. WALLICH ON THE PHYSICAL 



on deep-sea exploration down to the period in question. These 

 analyses, like the first of the above, are by the late Mr. David Forbes, 

 whose note on the subject I here transcribe : — " The specimens of 

 Atlantic mud which I have examined differ very essentially from 

 chalk in composition ; and no single one of them (if consolidated) 

 could be entitled to the appellation of chalk, as ordinarily under- 

 stood by geologists or chemists. In order to make a correct com- 

 parison 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, 



Folkestone. Sussex. 



Carbonate of lime 94-09 98-40 



Carbonate of magnesia 0-31 0-08 



Insoluble rock debris 3-61 1-10 



Phosphoric acid .traces. .... 



Alumina and loss iii analysis 0*42 



Chloride of sodium 1-29 



Water 0-70 



100-00 100-00" 



Eeferring to these, Mr. Prestwich observes, in a later part of his 

 address: — "From what I have previously said, you will have un- 

 derstood that, lithologically, there is but little resemblance between 

 the Atlantic mud and our typical white chalk, none that could 

 ever have led a geologist into any error of determination. In fact, 

 in no part of the area yet explored is there any thing at all to be 

 identified lithologically with the true white chalk. Even if it were 

 found that the superposition were conformable, the difference of 

 mineral character is too marked. At the same time it is to be 

 observed that the area of the Atlantic is so vast that, variable as the 

 deposit now going on seems to he, it is probably little, if any, more so 

 than that which went on in some parts of the Chalk series in the bed 

 of the Chalk ocean over the old European area. Of the rate of the 

 present deposit we know nothing. Is it even going on everywhere 

 over the deep Atlantic ? " 



Again, Mr. Prestwich says : — " The Atlantic abyssal mud has been 

 found to contain from 50 to 60 per cent, of carbonate of lime, 20 to 

 30 of silica, with small variable proportions of alumina, magnesia, 

 and oxide of iron. Its appearance, when dry, is chalk-like ; but it 

 is to be observed that our chalk is a much more homogeneous rock, 

 containing from 95 to 99 per cent, of carbonate of lime, while even 

 our grey chalk contains from 80 to 90 per cent. The large propor- 

 tion of calcareous Foraminifera in the chalk, and of siliceous Poly- 

 cystina and vitreous Sponges in the Atlantic mud, may, however, 

 render this rather a question of proportion than of radical difference." 



Lastly, I have to cite Sir Wy viUe Thomson's most recently formed 

 opinion on the subject, as expressed in his work ' The Depths of the 

 Sea ' (pp. 480, 481). " It would seem," he says, " from the analysis 

 of chalk, that siliceous organisms were entirely wanting in the 



