COREESPONDENCE. 
407 
advanced. Before explaining this theory I will allude to a phenomenon 
which is apparent in nearly every large exposure of flint-bearing chalk. 
Seams qfjlint, often not more than an inch in thickness (sometimes much less), 
may he seen in the chalk, runninf/ 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 
-0 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 
the bottom of the sea round a nucleus of any description, and then have 
been deposited with the chalk in such a position, 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. 
Bowerbank 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 
Eottiugdean, where many and striking examples may be seen. The 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 Henslow, 
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 vegetalale 
matters which had been .enclosed. In 
the process of drying, fissures would na- 
turally form in the chalk, in which the 
siliceous water could 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- ^ ... .j[ — ^ ~— r- 
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 whicli 
will better explain the occurrence of these continuous seams of flint. I 
enclose you a sketch of the cllft'with flint seams, and remain. 
Yours, etc., 
September Sfh, 1861. Silex. 
The Bed Sandstone Blocks of Dunmanway. 
Sib, — The beautiful representation of the Old Eed Sandstone block of 
Dunmanway, county Cork (given at page 248, ' Geologist' for JuTy) gives 
rise to the following suggestions 
Dunmauway's Old Red Sandstone bloclcs stand forth as rocks 
Of J/>a/'^?--markings ; not rubs and jolts of ice-blocks j 
Water went round to mark them. 
