150 MR. F. D. LONGE ON THE FORMATION OF FLINTS IN CHALK. 
soon effervesced fiercely with much heat, and the quantity of the 
material sensibly diminished. The effect of this acid on this 
powder was precisely the same as its effect on some pure silica in 
another tube, and upon ground flint in another tube. The 
evidence of these tests are I think conclusive, the inner part 
of the coating, although just as white as the chalk on the outside, 
Avas silica ; the same, or nearly the same, substance as the flint 
itself. 
The relation between the coating and the flint inside appeared 
to he that the Avhite powder next the flint in these nodules Avas 
silica in a granular state on its Avay to being converted into 
vitreous flint and added to the growing mass within. The nodules 
Avere flints in the process of formation, as Professor Sollas called 
them “incompletely formed flints.” 
An important point in the formation of flint, as shown in these 
nodules, is that it does not grow by the accretion of silica directly 
from the substance Avhen in a free solution, as in the case of 
crystals, but of the accretion of silica from the granular silica Avith 
Avhich it is immediately surrounded, and which must itself have 
been previously formed from silica in solution. 
But doubtless there was and must be moisture to assist in the 
transition of the silica grains into massive flint. It Avould appear 
that the formation of a flint involves three processes and as many 
problems. 
1. As to the presence of silica in a granular state in the chalk. 
2. As to the process by which the silica is separated from the 
other substance with which it is mixed, and collected together. 
3. As to the process by which it becomes converted into a hard 
mass of vitreous flint. 
It appears from the literature on the subject to which I have 
had access that the formation of flint and other similar mineral 
concretions is as yet an unsolved problem. 
With reference to the formation of flint, Professor A. H. Green, 
Avriting in 1876, says (‘ Geology for Students and General Readers,’ 
p. 141):- 
“We can only say . . . that the silica, which once pervaded 
the whole rock, has been separated out and gathered together into 
nodules ; Iioav this Avas done Ave cannot at present explain.” 
