194 



THE GEOLOGIST. 



in general is very small, would not be visible. Again, it is a well- 

 known ftict, 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 hoUovt^ed out ; the fossils having 

 thus disappeared. 



These small concretionary bodies are in most flints very easily 

 detected, from their having usually a very different aspect and colour 

 from the flint which surrounds them. Different 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. 



