A. J. Jukes-Browne—Relative Age of Flints. ol7 
that the decomposition of organic matter has played a prominent 
art. 
e Lastly, with regard to the vein-courses, I can confirm Mr. Abbott’s 
observation as to the frequency of two layers, and I agree with his 
conclusion that they must have been formed in fissures, for ] presume 
he means any kind of fissures, such as open joint-planes, and not 
necessarily faults. This being so, one would certainly have expected 
that the microscope would disclose some points of structural differ- 
ence between the flint of veins and that of nodules. Mr. Hill how- 
ever assures me that he cannot detect any material difference, and it 
would seem therefore as if we must admit that true flint can be 
formed in spaces which have been open cracks. It is quite possible 
however that the deposition of the silica in these cracks took place 
as soon as the cracks were opened, that is to say during the first 
upheaval of the mass and while the occluded sea-water was passing 
away from it. This water must have contained much silica in 
solution and the mere relief of pressure by upheaval may have been 
sufficient to cause precipitation of the dissolved silica, though why 
true flint and not chalcedony was deposited I am quite unable to 
explain. Similiar veins sometimes occur in the chert-beds of the 
Upper Greensand, but the silica in them is always chalcedony. 
If the flint-veins were formed after complete upheaval and by 
deposition from water percolating down from the surface of the land, 
it is difficult to understand when and under what conditions such 
water could first of all take up silica and then deposit it as flint. 
Such water would be much more likely to leave veins of calcite 
behind it, and calcite crystals do not unfrequently occur in empty 
shells of Ananchytes and Terebratula and in the cracks which often 
occur in the beds of “ Chalk Rock.” 
At the same time I admit that there are veins of flint which seem 
to have originated after the later disturbances of the Chalk, but the 
theory above suggested would still apply to such cases if we can 
assume that the areas in which they occur had not previously been 
raised above the sea level. 
There are also chemical questions which probably a little experi- 
ment would answer: imagine a mass of chalk for the first time 
upheaved above the sea-level, the sea-water with which it was 
saturated and which probably retained much silica in solution would 
sink to the parts below sea-level, and rain-water, taking up carbonate 
of lime, would percolate downward. Now what would happen 
when the carbonated water came in contact with that containing 
silica, would not the silica be precipitated ? 
In this paper I have only dealt with the general aspect of the 
question; the manner in which flint was formed is undoubtedly a 
great puzzle, and it will not be solved without much patient investi- 
gation. So far as I am aware flints only occur in very pure chalky- 
limestones, such as Cretaceous Chalk and the Portland Limestone ; 
but their occurrence in the latter shows that very deep water was 
not necessary for their formation. 
