HISTORY OF THE CRETACEOFS FLINTS. 81 



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 

 Poraminiferous 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 proba- 

 bly a not inconsiderable proportion of the inorganic silica which has 

 been found present in some specimens of the Atlantic mud is dis- 

 solved under the conditions that prevail. For, whereas the cal- 

 careous matter is furnished partly from the debris of Poraminifera 

 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, with the 

 solitary exception of the sjponges, subside to the bottom only after death, 

 this being equally true whether the Polycystina inhabit the entire 

 body of the ocean from its surface to its bottom, or live only near or 

 at its surface *. The result is, that the whole of the organic silica, the 

 moment it reaches the bottom, comes into contact with the pro- 

 toplasmic layer and is retained by it. Hence the quantity present in 

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

 by a mere dip into the superficial stratum of a few inches in depths 

 does not fairly represent the percentage of silica contained and sup- 

 posed to be equally distributed in the substrata, but only the accu- 

 mulated amount of that substance which has been getting accessions 

 for an indefinite period from the superincumbent waters. 



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 con- 

 ditions as have been here described, it is obvious that a period must 



* It will be seen on reference to my ' North- Atlantic Sea-bed,' p. 127, and 

 also in a paper on the Polycystina, in the ' Quart. Jo urn. Micros. Science' for 

 July 1865, that I then called attention to the fact that the AcanthometrcB, 

 which are abundantly represented in the surface-waters of the ocean, are not 

 purely siliceous, and therefore yield to the solvent action of acids, and eyea of 

 water, much more readily than any other siliceous-shelled Protozoa with which 

 we are acquainted. Not a trace of their siliceous remains is to be found either 

 in recent or fossil oceanic deposits. This fact has been completely verified 

 by Sir W. Thomson, in his work 'The Atlantic,' vol. ii. p. 340. 



Q. J. G. S. No. 141. G 



