194 Dr. Wallich on the Origin of the 



" A very important fact has to be here noticed in relation 

 to the siliceous materials which are supposed to be normally 

 and uniformly distributed throughout the substance of the 

 calcareous mud at the period of its deposition on the sea-bed. 

 In order to understand the full significance of this fact, it is 

 indispensable to recollect that, whereas the carbonate of lime 

 of the effete Globigerine and other fbraminiferous shells is to 

 a certain extent redissolved in the water charged with an 

 excess of carbonic acid, and the amount thus abstracted is too 

 insignificant to produce any material diminution in the mass 

 of the calcareous deposit, nearly the whole of the organic, and 

 probably a not inconsiderable proportion of the inorganic 

 silica which has been found present in some specimens of the 

 Atlantic mud, is dissolved under the conditions that prevail. 

 For, whereas the calcareous matter is furnished partly from 

 the debris of Foraminifera which pass their existence only at 

 the bottom of the ocean, and partly from such as live at the 

 surface and subside to the bottom only when dead, the whole 

 of the silex-secreting organisms, tcith the solitary exception of 

 the sponges, subside to the bottom only after death. The result 

 is, that the lohole of the organic silica, the moment it reaches 

 the bottom, comes into contact with the protoplasmic layer 

 and is retained by it. Hence the quantity present in every 

 sample of mud obtained [as all our samples hitherto have been) 

 hy a mere dip into the superficial stratum of a few inches in 

 depth J does not fairly represent the percentage of silica contained 

 and supposed to be equally distributed in the substrata, but 

 only the accumulated amount of that substance which has been 

 getting accessions for an indefinite period from the superincum- 

 bent tcaters. 



" In the case of the sponges that occur in such numbers on 

 every square yard of the calcareous mud, and live more or 

 less imbedded in the soft and luxuriantly developed nidus of 

 their own protoplasm, the result described must necessarily 

 take place in a still more signal degree, since every spicule, 

 and every particle of their siliceous debris, is not only formed 

 but accumulated within this protoplasmic environment. 

 Therefore, instead of there being from 25 to 35 per cent, of 

 silica, soluble and insoluble, in the calcareous mud, at a depth, 

 say, of eighteen or twenty-four inches below the surface there 

 is in all probability not more than is to be met with in an 

 average specimen of white chalk. 



" If we follow out to its legitimate issue a continuance of 

 such conditions as have been here described, it is obvious 

 that a period must arrive when the protoplasmic masses 

 (which, owing to their inferior specific gravity, always occupy 



