HISTORY OF THE CRETACEOTJS FLINTS. 85 



ancient Cretaceous sea. In chalk-mud, on the other hand, silica is 

 found in abundance, in most specimens to the amount of from 30 to 

 40 per cent. A considerable proportion of this is inorganic silica — 

 sand ; and its presence is doubtless due to the circumstance that our 

 dredgings have hitherto (1872) been carried on in the neighbour- 

 hood of land and in the path of currents A considerable 



proportion of the silex of the chalk-mud, however, consists of tJie 

 spicules of Sponges, of the spicules and shields of radiolarians, and 

 of the frustules of Diatoms. And tJiis organic silica is uniformly 

 distributed through the mass J' 



Now three distinct and important Assumptions demand attention 

 in connexion with these reports and analyses. 



In the first place, the assumption on which it will be remembered 

 a good deal of emphasis was laid by me at the beginning of this 

 paper — namely, that the chalk, as we now find it, gives on analysis 

 any thing like an accurate or, let me say, an even approximately- 

 accurate idea of the percentage of sihca, organic and inorganic, 

 present in it when it existed in the shape of calcareous mud at the 

 bottom of the old Cretaceous sea. 



In the second place, the assumption (for it is obviously quite im- 

 possible that it can be any thing more in the present state of our 

 knowledge, as to what may, or may not, be found a very few inches 

 beneath the surface of the muddy deposit) that a specimen of bottom 

 obtained from such a thii^ superficial stratum (where, according to 

 my hypothesis, nearly the whole of the silica is either dissolved or 

 stored up that has been gradually accumulating since the stratum 

 of nascent chalk deposited simultaneously with it) furnishes a trust- 

 worthy index to the lithological constitution of any portion of the 

 subjacent mass. 



And, in the third place, the assumption that the special analyses 

 of "Atlantic mud" which have been cited — and which are the only 

 ones heretofore published, so far as I can discover, that have guided 

 scientific opinion on the subject — furnish a fair indication of the 

 quantity of insoluble silica (" rock debris " of sorts) which is to be 

 found in samples of the typical mud procured from oceanic areas 

 sufficiently remote from currents capable of transportiug such debris 

 from volcanic or other regions. 



As regards one and all of these questions, I venture, for reasons 

 already adduced, to believe that generalizations have been formed 

 and relied upon which were based on data more or less inapplic- 

 able in each case, and were, consequently, in themselves faulty ; 

 and, further than this, with the most implicit faith in the absolute 

 accuracy of the analyses, both of my late friend Mr. David Forbes 

 and of Mr. Ward, I venture to affirm that, in the case of the former, 

 no more unfortunate and misleading example of " Atlantic mud " 

 could have been placed in the hands of this accomplished geologist 

 and chemist than that which was obtained from a position to the 

 south-eastward of EockaU. I know the ground from having tra- 

 versed it a little to the northward, in the ' BuUdog,' in 1860; 

 and being thus able to form an opinion as to the probable source, of 



