G.. Abbott —Contemporaneity of Chalk and Flints. 279 
formation from the pre-Silurian downwards, irrespective of the ex- 
ceedingly interesting eruptive series, which it is not the purpose of 
this preliminary notice to discuss. 
X.—Was THE Deposit oF FLiInT AND CmaLK ConTEMPORANEOUS ? 
By G. Aszorr, M.R.C.S. 
ANY difficulties remain to be solved and much work to be done 
before we rest satisfied that we can understand how flint 
took the peculiar shapes and position in which we find it in the 
Cretaceous strata. 
Most geologists will now agree with me that we must look to 
the action of segregation for the explanation. ‘The older theory of 
colloid silica, as well as that of the chemical replacement of the 
organic matter in the sarcode of sponges by silica, long held sway, 
and even yet their influence is to be traced in the most recent 
literature on the subject. 
To advance our knowledge and to obtain a working hypothesis 
at once accurate and comprehensive, ought we not to commence by 
trying to find out when the silica was deposited? Did it take place 
before or after the Chalk was raised above the sea-level ? 
Many writers imply, if they do not state, that the formation of 
flint took place contemporaneously with the deposit of the chalk 
strata. With this I disagree; anyway it seems of the utmost im- 
portance that this point should be inquired into, for manifestly the 
influences at work to effect the chemical changes would be quite 
different in the two cases. 
The tabular flint of the Upper Chalk appears to give us some 
valuable data with regard to this point. 
During the last twelve months I have examined all the specimens 
I could find, many in situ, and they all showed signs of having 
“crown” in faults." Besides being found in the position where one 
would expect to find such fissures, they show innumerable traces of 
having been formed of two plates originally distinct, but which by 
the gradual addition of silica have approached each other and united 
into one thick tabular mass. Few specimens fail to show where 
this junction took place, whilst here and there irregular cavities 
remain, due, I presume, to the process in that portion being incomplete 
when segregation ceased. 
‘The thickness of these tabular flints varies considerably —ranging 
from 4” to 6” or more, sometimes almost reaching these extremes 
within the space of a foot, but more often lying for long distances 
between fairly parallel walls. The external surfaces correspond to 
those of the broken chalk, having here and there excrescences which 
indicate the growth of flint in cavities on the walls of the fissure, 
the surfaces in some specimens being so angularly rugged that the 
Chalk must have been dry and hard when the fissure occurred. 
I have looked for, but hitherto have failed to find, any other satis- 
factory explanation of these remarkable dyke-like deposits. 
1 This can hardly apply to the continuous horizontal layer of flint which occurs 
in the Chalk from Thanet to Dover.—Epir. Geou. Mac. 
