398 GEOLOGY OF THE SOUTH EAST OF DORSETSHIRE. 



at Tervureen, in the Forest of Soignies, I have found pebbles 

 perfectly similar in every respect, in the former case occupy- 

 ing a position in the midst of the beds which are called 'cal- 

 caire grossier,' and are assumed to be the equivalent of the 

 London clay. 1 I mention these localities, because I think 

 that the origin of these pebble-beds, and their particular po- 

 sition in the supra-cretaceous deposits, have not received all 

 the attention which they deserve. An observation upon the 

 subject will not be out of place here. 



These pebbles invariably occur either in the lower beds of 

 the plastic clay, or as imbedded in the pudding-stone of Hert- 

 ford and Dorsetshire, or as in loose aggregations of superfi- 

 cial drift. In both the latter cases, their original position as 

 pebbles was doubtless that of the former. Now it is the opin- 

 ion of several eminent geologists, that the plastic clay beds 

 have been derived in great measure from the destruction, at 

 a very early period, of the lower green sand formation, which 

 is the expressed idea of Professor Phillips, for instance. 2 — 

 ('Treatise on Geology,' pp. 165 — 169). And in considering 

 the origin of these pebbles, I have been led to believe, that 

 though they are called flint (Blackheath flint), they are not 

 flint but chert, and derived from the chert beds of the green 

 sand. Dr. Mitchell (' Mag. Nat. Hist' vol. ii. p. 218) has 

 very carefully described the true character of these " peculiar 

 flints," as they occur in the neighbourhood of London, and I 

 am sure, after that elaborate detail on the subject, no doubt 

 can remain, that they are not flint, but chert pebbles. There 

 is nothing extraordinary in admitting such an origin ; — it can- 

 not be the " countless multitudes" and " millions" of them 

 which startle the mind ; — for it is a fact, that nearly -^gy of 

 all the drift gravel in that part of Dorsetshire of which I am 

 treating, is composed of chert, which I have traced to the 

 green sand of Devonshire : so that even in modern times, 

 compared with the plastic clay era, the chert beds of the green 

 sand have furnished the materials for innumerable beds of 

 pebbles, of much greater extent than those composing the 

 Blackheath and Croydon deposits. Analogous to the suppo- 



1 It is from this, and other causes, that I am not satisfied with the ar- 

 rangement which assigns to the Brussells beds the geological cognomen 

 of ' calcaire grossier.' Mr. Morris has, however, given reasons for us to be- 

 lieve, that the plastic clay and London clay are of the same formation, al- 

 though of different divisions — (see Geol. Pro. vol. ii. p. 450), — an arrange- 

 ment of some importance. 



2 Mr. Lyell, however, speaks of " shingle composed of perfectly rolled 

 chalk-Jlints, with here and there small pebbles of quartz." — Geol. vol. iv. p. 

 212. 



