W. A. Richardson — The Origin of Cretaceous Flint. 535 



upturned Wealden, Purbeck, and Kimmeridge strata, and proves that 

 there had been produced in pre-Gault times a set of flexures wholly- 

 independent of those of post-Oligocene age, though parallel to them. 

 These earlier flexures are ignored by the rivers. 



Mupe Bay, east of Lulworth Cove, affords a clear view of the 

 passage of the Purbeck beds up into the Wealden, and of the abrupt 

 but conformable junction of the Lower Purbeck and Portland Stone. 

 Half a mile east of Lulworth Cove a ledge of the cliff provides an 

 unrivalled opportunity of examining the lower part of the Purbeck 

 beds, including the junction with the Portland Stone, the thin 

 layer of carbonaceous gravelly soil known as the dirt-bed, numerous 

 stumps and prostrate trunks of coniferous trees silicified and enclosed 

 in calcareous tufa, and the brecciated limestones associated with 

 tufa, known as the " broken beds". Here the incoming of bands of 

 tufa, among the sedimentary limestones, and the close association 

 of such incoming with brecciation of the limestones, can be studied 

 in detail. Westward from Lulworth Cove the cliffs illustrate the 

 intense compression and supplementary overthrusting which all the 

 formations have undergone in the neighbourhood of the Isle of 

 Purbeck fault. 



III. — The Origin of Cretaceous Flint. 



By W. Alfred Richardson, M.Sc, B.Sc. (Eng.), F.G.S., A.M.I.Min.E. 



1. Introduction. 



IT will be recalled that to the list of theories relating to the 

 origin of flint Liesegang 1 has added another, namely, that the 

 flint is due to the rhythmic precipitation of a silica solution diffusing 

 through the Chalk. Cole 2 in an admirable essay has made this 

 view accessible to English readers, and at the same time somewhat 

 expanded the original suggestion. Yet little by way of evidence is 

 offered beyond a certain plausibility in the idea, and its undoubted 

 competence to explain better than any other hypothesis yet 

 suggested the remarkably regular recurrence of flint lines. When 

 reading over the chief Cretaceous literature with this problem in 

 mind, there seemed to me to be a not inconsiderable body of fact 

 lending support to this view. Accordingly the object of this paper 

 is to examine existing data in the light of Liesegang's suggestion in 

 order to see whether or not it may be regarded as a reasonable 

 working hypothesis. 



There are three outstanding questions connected with flint-origin 

 which still await decisive answer. They are, namely : — 



1. The age of formation relative to the Chalk. 



2. The source of the silica. 



3. The cause of the regular recurrence of flint bands. 



This paper is only indirectly concerned with the first of these 

 questions, and I shall, therefore, only summarize the position with 

 regard to relative age and pass directly to a discussion of the 

 remaining questions. 



1 R. Liesegang, Geologische Biffusionen, Dresden and Leipzig, 1913, p. 126. 

 a G. A. J. Cole, Geol. Mag., 1917, pp. 64-8. 



