194 
THE GEOLOGIST. 
in general is very small, would not be visible. Again, it is a well- 
known fact, that many organic remains in flint have been removed 
by natural causes, leaving very little or no trace of their former 
existence, so that, in this latter instance, the centre may be exposed 
without any of the original organic structure being discoverable. 
There is an interesting specimen, from Muswell-hill, fig. 2, having 
two of the nodules with the centres hollowed out ; the fossils having 
thus disappeared. 
These small concretionary bodies are in most flints very easily 
detected, from their having usually a very diflerent aspect and colour 
from the flint which surrounds them. Diflerent specimens vary in 
thickness, and some are opaque, others semi-transparent. 
On looking over a gravel-heap, flints are occasionally met with 
having rough and irregular external surfaces. On such surfaces I 
have detected many of the pea-like concretions above alluded to. 
These have generally the upper or most exposed part flattened from 
attrition, and occasionally the concretions are so much water-worn as 
to expose the fossil in their centres. Sometimes these small bodies 
are found partly rubbed down on the flints with a smooth exterior. 
Although I have drawn particular attention to the fact that many of 
these bodies are of the size of a pea, I should observe that much 
larger, as also very minute, examples occur. They vary also in shape 
from spherical to oviform and subcylindrical. 
I am induced to make this communication, as it bears upon the 
subjects noticed in my two recent papers read before the Geological 
Society respecting the organic centres of the nodular concretions in 
the London clay, and on the origin of the structure of some banded 
flints. 
