478 Correspondence—Mr. G. Abbott. 
fissures, and that microscopically it has the same appearance as flint 
from the Nodular layers. At present I must confess to being 
ignorant of his reason for putting flint floors in a separate classifica- 
tion. Is it simply their wide extent? True, they are generally 
quite solid and free from the cavities so common in the more vertical 
sheets of flint, but this, I had supposed, was due merely to the 
horizontal position favouring the deposit of silica because of the 
slower current of water. 
These horizontal layers, whatever their origin, show, with this 
exception, the same signs as the others of having been formed 
originally of two, plates, and probably in open joint-planes. Should 
Mr. Jukes-Browne’s suggestion that the flint was deposited in 
cracks during the first upheaval of the chalk be correct, we must 
believe that these cracks often extended at least through 100 ft. of 
soft chalk saturated with sea-water, and that many of these fissures 
were far from being vertical, frequently at an angle of only 30° 
with the horizon. 
My suggested explanation of the absence of flint in the Lower 
Chalk only referred to the beds in our 8.E. district, and I take it 
that Mr. Jukes-Browne’s experience does not refute my belief that 
wherever there are nodules in the Lower Chalk they are also present 
in the Upper Chalk, but not vice versa. He is doubtless aware that 
it has been asserted that the total percentage of flint in different 
beds is roughly the same. If this be true, surely we must look to 
segregation as the cause of all the visible flint in the cretaceous 
strata, and the question is, when did this process take place ? 
The varying character of flints in successive zones is doubtless of 
importance, but does not prove that flint was contemporaneous with 
the chalk; though evidence of the traces of the soft parts of organisms 
in flint would probably be conclusive, and I should be glad to hear 
if such have been found. 
The cores I spoke of are not tubular, and I still fancy are not 
formed round Doryderma, though probably confined to one particular 
zone. ‘The outer coats of these flints are generally shaped like a 
potato, hollow inside, with a solid cylindrical core passing through 
from end to end. This is the simplest form, but sometimes other 
cores cross the first at various angles. I should be glad to submit 
specimens to Mr. Jukes-Browne if he would care to see them. 
These cores, as well as some other flints I have, containing re- 
markable loose kernels, lead me to ask him to allow me to add 
‘ Mechanical Flints”’ to his classification, and to include concentric 
circles of flint with his Paramoudra division. 
I can only briefly touch on the chemical influences involved, 
except that I doubt that Mr. Jukes-Browne’s theory of the pre- 
cipitation of silica through contact with carbonated water can be 
accepted; but his paper was all too short, and I should be glad if 
in some future communication he would give us his views as to the 
sources whence the large quantity of silica for Flint-floors, etc., was 
derived. . Grorce Aspport, M.R.C.S. 
TUNBRIDGE WELLS, Aug. 1, 1898. 
