Vol. 62.] THE CIAY-WITH-FLINTS. 139 



IY. Products ojf the Solution oe Chalk. 



From the known purity of the Upper Chalk, it will readily be 

 granted that, if the Clay-with-Flints has been mainly derived from 

 the dissolution in situ of Chalk-with-Flints, a very great thickness 

 of this chalk must have been dissolved in order to furnish the 

 material of the clay. 



It is well known that the soft limestones of the Upper Chalk 

 contain a very small proportion of insoluble matter ; but, until 

 recently, very few accurate analyses of these chalks existed, and 

 even now hardly sufficient exist to furnish data for estimating the 

 average amount of clay to be found in each zone of the Upper 

 Chalk. Moreover, the determinations of the amounts of insoluble 

 residue which have been made by Dr. W. F. Hume, F.G.S., and 

 Mr. W. Hill, F.G.S., show that in some zones these amounts vary 

 in different parts of the country. This seems to be more especially 

 the case with the lower zones, namely, those of Micraster cor- 

 testudinarium and M. cor-anguinum. 



I have, therefore, thought it desirable to depend, as far as possible, 

 on analyses of samples obtained from places within the area of 

 which this paper treats. In order to increase the number of such 

 analyses, my friend Mr. W. Hill has kindly ascertained the amount 

 of insoluble residue in several fresh samples of chalk from the zone 

 of Micraster cor-anguinum, and in one from that of M. cor-testudi- 

 narium. For two of these samples I am indebted to Mr. H. J. 

 Osborne White, F.G.S. ; another, from the zone of M. cor-anguinum 

 at Winnal near Winchester, was sent by Mr. Charles Griffith, F.G.S. ; 

 and the fourth was obtained by Mr. W. Hill himself at Knebworth, 

 south of Stevenage in Hertfordshire. 1 



Combining the results of these analyses with a selection from 

 those previously made and recorded, 2 we get the following averages 

 for four of the zones : — 



(1) RESIDUES IN THE ZONE OF MlCRASTER COR-TESTVDIXARIUM. 



Grammes. 



Chalk from Stourpaiue (Dorset) 248 



,, „ Lewes (Sussex) O66 



„ Dover (Kent) 0-61 



„ ,, Remenhani (Berks) 1*43 



,, ., Medmenham (Bucks) 3*72 



„ Hitchin (Herts) 0'88 



6)9-78 = average of 1-63. 



1 It should be mentioned that the analysis of a sample referred to Hitchin 

 in vol. iii of the Geological Survey-Memoir on the Cretaceous Rocks of Britain 

 (p. 320) was really of chalk obtained at Knebworth. 



2 See 'The Cretaceous Rocks of Britain' Mem. Greol. Surv. vol. iii (1904) 

 pp. 309 et seqq. 



