182 The Qeohgy of the Berks ^ Hants Extension, 



greater ; but other forms of land surface probably attended tbe 

 formation of potholes and Clay-with-flints.^ 



A vast time must be allowed for the formation of Clay-with-flints 

 from Chalk-with-flints. But the drift covering tbe high grounds, 

 and capping some of the highest points, is very ancient, dating 

 from an earlier period than the Boulder clay. The chalk valleys 

 and coombs, and the Vale of Pewsey, have all been excavated since 

 its deposition, and the excavation of the latter to nearly its present 

 depth, and its elevation above the sea, had taken place before the 

 period when the mammoth left its remains in the valley. To this 

 latter period we cannot yet count back by thousands of years. 

 The remaining cuttings near Marlborough are through Upper- 

 chalk, the fossils in which are few and fragmentary. In the cutting 

 near Granham there were many funnel shaped sponges enclosed in 

 the flints, and at Marlborough Station masses of the branching 

 sponge {Spongia ramosa) were found. 



The following list comprises all the other fossils observed. 



Cidaris sceptifer (spines) 



Bourgueticrinus ellipticus (fragments) 



Qoniaster 



Micraster con-anguinum 



Ananchytes ovata 



Galerites alho-galerui 



Terehratula carnea 



Inoceramua Brogniartii 



Lima Hoperi 



Spondylus spinosus 



Pleurotomaria perspectiva 

 Note. — Since the above suggested explanation of the formation of Clay-with- 

 flints was wi'itten, I have found in the Memoir illustrating sheet 7, of the 

 Geological Survey, published last summer, that Mr. Whitaker had already come 

 to the same conclusion as to the origin of that formation. He does not however 

 claim it as a new suggestion, but gives a quotation from the Journal of the 

 Geological Society of 1851, in which it is only mentioned as a prevalent assump- 

 tion to be condemned, by Mr. Trimmer, a strong advocate for the mechanical 

 origin of potholes in chalk. 



III. The Yale of Pewsey. 



The Vale of Pewsey is the largest of the three indentations in 



the great escarpment of Chalk which runs across England from 



Dorsetshire, through Wiltshire, Berkshire, &c., into Yorkshire. 



' See Mr. Prestwich's valuable paper. Quart. Jour. Geolo. Soc. vol. 11. 



