148 Dr. PiTTON on the Strata below the Chalk. 



unchanged^ and still exemplify the power of geological causes in modifying 

 the civil condition of countries, as well as their external features. 



In the bottom of the " Punch Bowl," perhaps 200 feet beneath the road, a small stream arises, 

 and the north flank of the highest ridge of Hindhead, which runs nearly west from the summit, is 

 furrowed by nine or ten similar and nearly parallel trenches : the deepest of these is called Hack- 

 ham Bottom, the stream from which is continued along the verge of Thursley Common to the 

 Wey at Elstead. The ridge represented in the section No. 4. is that of Grayshot Down, as it 

 appears from the lower ground on the north. It declines westward to Harem, and also to the 

 south ; so that there is here distinctly an antichnal line, more than three miles north of the 

 central valley of the Weald between Ludgershall and Harting-coml^e. 



(63.) Although Hindhead is the most prominent point in this part of the 

 country, it is not, strictly speaking, the escarpment of the Lower green-sand ; 

 for the strata, which rise very gradually on the west of the denudation, run out 

 to the east beyond the highest summit, and caps of sand are preserved on some 

 of the advanced ridges and in several detached points. For the same reason, 

 Blackdown Hill, about four miles south of Hindhead, is no more than a 

 massive cap of sand, resting with a very slight inclination upon the Weald 

 clay : and all the deep ravines which separate the heights and ridges on the 

 west and north-west of Blackdown, towards Lynchmere and Haslemere, 

 have sand only at top ; the clay rising here to not much less than six hun- 

 dred feet above the sea. Blackdown forms the north-eastern promontory 

 of what may be called the central valley of the denudation ; the opposite and 

 converging side of which is a similar escarpment of the Lower green-sand, 

 extending from Harting-combe to Bexley-hill. The floor of the narrow 

 intermediate space consists of the Weald clay, which, close to the junction, 

 abounds in Cypris Faha ; and where the valley opens into the wider expanse 

 of the Wealds, the first range of the Wealden sand and sandrock, described 

 by Mr. Martin, about 25 feet thick, and at a vertical distance of about 200 

 feet from the top of the clay, forms a sort of bar, or elevated ridge, across the 

 entrance; occupying an extensive surface between Ludgershall upon the Lod, 

 and Northhurst, and being thence continued, at a corresponding distance from 

 the top of the clay, all round the denudation. The course of the Lower 

 green-sands on the south and south-west of this valley having been pointed out 

 by Mr. Murchison, it is unnecessary to pursue this description farther. 



(64.) Weald Clay. — The lowest beds of the tract near Guildford have 

 recently been exposed in a pit for the extraction of gravel at Pease-marsh, 

 an extensive flat on both sides of the London road, between Arlington, 

 south of Guildford, and Godalming. On examining the place, to which 

 my attention was called by Mr. Murchison, I found beneath a thick- 

 ness of four or five feet of flint gravel and sand^ distinct strata of blue clay. 



