and Marlborough Railways. 181 



bed into irregularities in the dissolving chalk. Everything here 

 also favours the supposition, that the origin of theClay-with-flints 

 is to be ascribed to the gradual dissolving away of the Chalk- with- 

 flints under a capping of drift Brick-earth. The flints, representing 

 a much greater thickness of chalk than the 6 feet into which they 

 are now crowded, have subsided gradually, without losing any of 

 the irregularity of their forms. The overlying drift or Brick-earth 

 has also sunk, the clay and sand and a few small tertiary pebbles 

 finding their way around and below the flints as the chalk enclosing 

 them was dissolved. Large masses like the Sarsen stones would 

 of course remain above the flints, subsiding among the folds of the 

 Brick-earth on the failure of support below. Mr. Whitaker, in 

 describing these deposits of Clay-with-flints and Brick-earth, says 

 of thern^ that the Brick-earth is generally underlain by the Clay- 

 with-flints — that the Clay-with-flints never occurs ofi" the Upper- 

 chalk — that the Tertiary beds and the Lower-chalk are free from it — 

 that it is not found in the bottom of chalk vallies, but that it often 

 extends someway down their sides. All these facts are quite con- 

 sonant with this mode of formation. The Clay-with-flints must 

 underlie the Brick-earth, when the latter is present, but the whole 

 of it may be absorbed into the Clay-with-flints. Upper-chalk- with- 

 flints must be the under-lying formation to furnish the flints ; and the 

 Clay-with-flints, resulting from drift covering the higher ground, and 

 older than the chalk valleys, will not be found in the bottoms of the 

 latter ; but it may extend down their sides, where, as at Warnhams 

 the thickness of the drift is great. 



A covering of drift made up of Tertiary materials, seems 

 greatly to promote the formation of potholes, and the irregular 

 erosion of the chalk ; Tertiary beds, unless where they thin out 

 appear to protect it. Perhaps in consequence of their more 

 regular stratification, they do not give so ready an access to the 

 acidulated water as the drifts do. On both Tertiaries and drifts, 

 the growth of vegetation is much more luxuriant than on the 

 bare chalk, and the supply of carbonic acid proportionately 



' Memoirs illustratiag sheet 13 of the map of the Geological survey, page 54. 

 Quart. Jour. Geolo. Soc, vol. 18, page 265. 



