270 Dr. FiTTON on the Strata below the Chalk. 



itself, may have originated in a preexisting fissure, by which the direction 

 of the principal stream may have been determined, and the effect of the 

 denuding- waters subsequently modified. 



My observations were resumed, on the north of Swindon, in the tract 

 around Hazeley and Garsington ; and the country thence through the con- 

 tiguous counties of Oxfordshire, Bucks, and Bedfordshire, as far as Wo- 

 burn, is so much alike in geological structure and composition, that the 

 whole may with advantage be considered together. I shall therefore first 

 state generally the order and composition of the strata, and then illustrate 

 the sections PI. X. a., Nos. 18, 19, 20, and 21, with some details derived 

 from points where the beds are best exhibited. The recent sheets of the 

 Ordnance survey, Nos. 13, 45, and 4G, are so well executed as to furnish all 

 the assistance that can be desired in the examination of this part of England: 

 and their aid will be almost necessary in perusing these remarks*. 



Chalk. — The most striking feature in the outhne of the chalk, north-east of Swindon, is the 

 sudden retreat of the Downs above referred to ; by wliich the escarpment is thrown back towards 

 the south between Woolstone, on the west of Wantage, and Watlington, not less than eight miles. 

 The subsequent direction of the outcrop is nearly parallel to that of Marlborough Downs, and 

 through its whole course thence into Norfolk, the lower chalk and chalk-marl run out far 

 beyond the escarpment of the upper strata, which, near Nuffield, on the main road from London 

 to Oxford, through Nettlebed and Abington, is more than two miles from that of the chalk- 

 marl at Gould's Heath. On the lines through Stoke Talmage and Tetsworth, the distance is 

 nearly four miles ; and the space varies between one and four miles all along the range, thence 

 to the north-east of Dunstable, becoming still more considerable in the lower tracts of the 

 east of Bedfordshire and Cambridgeshire, where the beds are less inclined. 



The angle at which the strata in this part of England rise, in general towards the west of 

 north, in many cases does not exceed 1° or 2°; and the contiguous beds are frequently so mixed 

 at their junction, as to render it very difficult to define them. An additional source of obscurity 

 occurs between Aylesbury and the road from Dunstable into Bedfordshire ; where large accumu- 

 lations of superficial gravel (comparatively recent, since it includes fragments of chalk,) mask 

 the substrata, or totally conceal them. A remarkable deposit of the same kind in the south and 

 south-west of Oxford, is mentioned by Mr. Conybearef ; and Dr. Buckland has made the trans- 

 ported matter of the Valley of the Thames, the subject of a paper in these Transactions, in which 

 his views of the Theory of its production are fully explained J . 



Upper green-sand. — This formation is not generally so prominent in this tract, as in the coun- 

 ties near the coast ; but its representative is found along the base of the chalk marl in all the 

 places which I myself examined. Thus, on the line of the Abington road to Oxford, it appears 

 on the descent from Gould's Heath towards Bensington ; and firestone, I was informed, is obtained 



* The authorities connected with this tract are, the general maps of Mr. Smith and Mr. 

 Greenough, and the Sections and County Maps of the former. The only published description 

 of the sti-ata is that of Mr. Conybeare in the "Outlines of England and Wales", p. 170. — 183, 

 from which I have throughout derived most valuable assistance. 



•j- Outlines, &c. p. 190. X Geol. Trans., 1st Scries, vol. v. p. 51G — 544. 



