COLE : CHALK OF YORKSIIJEE. 
25 
the nodular form, of great thickness, but not wholly composed of 
flint ; that is to say, streaks and patches of chalk are imbedded in 
the flint. Examples may be seen in several quarries in a straight 
line between Wetwang, North Dalton, and Middle ton. This 
horizon is styled by Mr, Blake (Proceedings, Geol. Association, 
Jan. 1 878) the " Zone of immature flints ". He C(nisiders, it seems, 
that the process of the development of flint was arrested by the 
elevation of the chalk, and removal of pressure. It may be so, 
but the whole question of the formation of flint in chalk is still 
suh jfidice. 
Above these last lies the flintless chalk, but curious to say, 
though there are no flints, the chalk, on analysis, yields nearly 
twice as much silica as the chalk which has flint. The silica seems 
to be dispersed throughout the mass, instead of being aggregated 
in tabular or nodular patches. 
Now what is the origin of all these various forms of flint, 
including the huge " paramoudra" or " potstones "' which are met 
with in the cliff at Flambro' and Speeton, 3 to 4 feet in height, 
and i to 2 feet in diameter ? 
One thing is clear, that, in a large majority of cases, they 
represent a mass of silica, which, by attraction or affinity, has 
been absorbed from the sea water which contains a quantity of 
silica in solution, round a nucleus of some decaying animal or 
vegetable. 
Siliceous sponges, whose nutrition is contained wholly in the 
seawater, which passes through their pores, naturally form a basis 
for the accumulation of flint, and accordingly we find many flints 
bearing the exact shape of cup-shaped zoophytes, such as spongites 
and ventriculites. But in addition we nut unfrequently find such 
a form as an Echinus, or " Sea Egg" completely transformed into 
flint, the external rays being as clearly delineated on the surface, 
as in the living specimens. Here, the decaying animal was evi- 
dently replaced atom by atom by silica. 
