68 Dli. WALLICH 02J THE THYSICAL 



7. A CONTRIBTJTION tO the PHYSICAL HiSTORY of the CrETACEOTTS 



Plints. By Surgeon-Major Wallich, M.D. (Read December 

 17, 1879.) 



(Communicated by the President.) 



Some years ago, whilst endeavouring to clear up the obscure points 

 in the history of the Cretaceous flints, I naturally turned for guiding 

 data to ;such analyses as were forthcoming of the Chalk and Globi- 

 gerine ooze of the Atlantic. But the further my inquiries were 

 pursued in this direction, the stronger grew my conviction that 

 no more fallacious test of the percentage of silica originally present 

 in the White Chalk could be resorted to than that of assuming as 

 a standard the percentage it now contains, and hence that any 

 comparison of the calcareous mud with the ancient Chalk, instituted 

 with the view to determine this percentage, must necessarily prove 

 equally fallacious. 



This result, however, was only to a certain extent unlocked for, 

 inasmuch as I had long previously suspected, on entirely distinct 

 grounds, that the almost complete absence of silica in the flint-bear- 

 ing Chalk did not arise from any deficiency in it of that substance 

 whilst it was yet in a plastic state at the bed of the ancient sea, 

 but was due to certain special conditions, which led not only to the 

 continuous elimination of the siliceous material for a time mechani- 

 cally associated with the calcareous mud, but to its consolidation in 

 the stratified layers alternating with the Chalk, which constitutes 

 by far the most striking and, at first sight, unaccountable feature in 

 this formation. 



' In directing attention, at the outset of my observations, to the 

 writings of those who have preceded me in this line of inquiry, I 

 am actuated by two considerations, namely, a desire to show how 

 much remains to be done before our knowledge of the Chalk flints 

 can be regarded as even approximately complete, and to leave no 

 ambiguity as to the purport of my own investigations, in so far as 

 they can be regarded as original. 



Twenty years ago Mr. Man tell, whilst epitomizing the works of 

 M. d'Archiac, Mr. Bowerbank, and others, described the nodules and 

 veins of flint that are so abundant in the Upper Chalk as having 

 been probably produced by the agency of heated water holding 

 silica in solution. The perfect fluidity of the siliceous matter before 

 its consolidation he considered proved, not only by the sharp moulds 

 and impressions of shells &c. retained by the flints, but also by the 

 presence of numerous organic bodies in the substance of the nodular 

 masses, and the silicified condition of the Sponges and other "Zoo- 

 phytes" which swarm in some of the Cretaceous strata. The solvent 

 power of superheated water on rocks containing silex might there- 

 fore be fully adequate to produce all the phenomena presented by 

 the nodules, dykes, veins, &c. of the Chalk formation ; and the 

 chalk flints might possibly, he thought, have originated from the 



