MU. F. D. LONGE ON THE FORMATION OF FLINTS IN CHALK. 151 
In another passage he says (p. 174) : — 
“ It has been observed in laboratory experiments that when 
different substances in a state of fine division are mechanically 
mixed together, certain of them do separate out and congregate 
together in nodular masses. ... It is usual to speak of this as 
Concretionary Action. . . .” 
It is clear that in both these passages Mr. Green is speaking of 
the substance of which mineral concretions are formed as mineral 
matter, not in complete solution, but in a finely divided granular 
state, and there seems to be good ground for his view that silica in 
this form is more or less abundantly disseminated in all chalk 
formations, except so far as it has been separated out and collected 
in concretionary nodules. 
As to the origin of the granular silica in the chalk, there seems 
to be a general concurrence of opinion that its presence is due to 
the extraction of silica in solution in the waters of the cretaceous 
sea, by sponges, etc., and its deposition on the sea floor from the 
decomposition of these organisms. 
In considering the problem of the separation of the granular 
silica in the chalk rock from the chalk and other mineral substances 
with which it is mixed, we may assume, I think, that the silica, 
though not in solution, was in suspension in the water which 
percolates the chalky mass, so as to be moved about with the 
water. 
But what is the cause of its being separated from the other 
substances in the water and collected together \ 
This phenomenon would seem to imply the action of some 
attractive force bringing together particles of the same substance, 
to the exclusion or displacement of other substances with which it 
is mixed. 
The attraction or cohesion of matter has been described as being 
analogous to the action of a magnet in attracting and holding iron 
in cohesion ; but I have not met with any authority for attributing 
concretionary formations to magnetic or electric action. The force 
at work still lies hid in the great mystery of physical science — the 
problem which Newton left for future generations to solve— the 
problem of attraction of matter to matter, the manifestations of 
which are familiar facts of ordinary observation, as the cohesion of 
matter, the force of gravity, capillary attraction, etc. 
