corhespondence. 



407 



advanced. Before explaining this tlieory I will allude to a phenomenon 

 which is apparent in nearly every large exposure of flint-beariiig chalk. 

 Seams qf jiint, often not more than an inch in thickneHs (sometimes much less), 

 ■may he seen in the chalk, runninc/ at almost right angles to the lines of strati- 

 fication. From these seams it is possible to detach slabs some feet in super- 

 ficial area, but only an inch or so in thickness. Now, I ask any person 

 \A ho has the slightest knowledge of the nature of deposition and stratifica- 

 tion, whether such a mass as I have described could have been formed at 

 tlie bottom of the sea round a niu'leus of any description, and then have 

 been deposited with the chalk in such a ])Osition, viz. at a considerable 

 angle to the lines of stratification. I think that the answer will be in the 

 negative, and that we must have some other theory than that held by Dr. 

 Bower bank to account for the presence of, at any rate, such masses as 

 these. You have probably noticed the seams, to which I allude, yourself, 

 as they are abundant everywhere along the south-western coast of England. 

 My atiention was first drawn to them in the cliffs between Brighton and 

 Kottingdean, where many and striking examples may be seen. Tiie only 

 theory which can explain these occurrences, and which is sound as far as I 

 can see in other respects, was held, I believe, by the late Professor Ilenslow, 

 who considered that the chalk flints had been formed after the deposition 

 of the calcareous stratum in which they are found, during its elevation 

 from the sea, by the percolation and concentration of water, highly charged 

 with silica, into cavities left by the decomposition of animal and vegetable 

 matters which had been enclosed. In 

 the process of drying, fissures would na- 

 tui-ally form in the chalk, in which the 

 siliceous water coidd accumulate and 

 leave its deposit of flint, in the same 

 manner as it had accumulated and formed 

 " nodular " masses in the cavities left by 

 the decomposition of sponges, echino- 

 derms, mollusks, etc. I think it is also probable that the gases formed 

 by the decomposition of these organic bodies would enlarge the cavities 

 formed in the yielding chalk; and might not some chemical affinity or 

 exchange also have assisted in the elimination of the siliceous particles 

 from the water? 



I should be glad to hear from yourself or correspondents, either facts 

 which will tend to support this theory, or some other hypothesis which 

 ^^ ill better explain the occurrence of these continuous seams of flint. I 

 enclose you a sketch of the cliff with flint seams, and remain, 



Yours, etc., 



Septemher 8th, 1861. Silex. 



The I^cd Sandstone Blocks of Dunmamvay. 



Sir, — The beautiful representation of the Old l\ed Sandstone block of 

 Dunnumway, county Cork (given at page 218, ' Geologist' for July) gives 

 rise to the following suggestions : — 



Dunmanwjiy's Old Red Saiulslonc blocks stand forth as roclcs 

 Or ?<;rtA'r-niar]<ing:s ; not rubs and jolts of icc-blocks ; 

 Water went round to mark them. 



