284 Dr. PiTTON on the Strata below the Chalk. 



more than 600 feet. The summit level of the Grand Junction Canal at the 

 opening of this vale to the north-west, is 404^ feet above the sea : and a 

 branch of the canal is conducted at the same level along- the foot of the 

 chalk range to Wendover. The line of Section No. 20. runs from a point 

 about two miles west of that town, above which Combe Hill, or (as it is called 

 in the Ordnance Map) Wendover Hill, rises to 903 feet above the sea ; and 

 thence the ground descends rapidly towards Aylesbury, the height of that 

 place being about 380 feet above the sea*, and a few feet above the flat 

 occupied by the Kimmeridge clay on the north-west of the town. The slope 

 and undulating ground from the chalk range to Aylesbury are occupied by 

 the strata between the chalk and the Portland stone ; and the corresponding 

 tract, from Thame on the south-west towards Leighton on the north-west, 

 which is emphatically called "the Vale of Aylesbury ", consists of the same 

 strata. The Portland stone forms a distinct ridge from Hartwell to Scots- 

 grove, on to the road from Aylesbury to Thame, of which the highest point 

 near Dinton Castle is not much inferior in elevation to many parts of the 

 chalk range. Between Aylesbury and Leighton the first rise of the Portland 

 beds is obscured by accumulations of superficial gravel. 



The Vale of Aylesbury in general has been lowered and levelled by the 

 agency of water; but several detached portions of the beds between the 

 Gault and the Portland stone, are still found on the higher parts of the ridge 

 about Stone, and on the summits of the insulated heights beyond it. The 

 strata seem to increase in thickness in approaching their outcrop, but this 

 enlargement is probably no more than apparent, and produced by their 

 better preservation near the most elevated points. If these places were once 

 detached summits, or islands, in a shallow sea, of which the chalk escarp- 

 ment formed the shore, the waves would naturally wear down and carry 

 away the strata in the lower ground, while the portions above the reach of 

 their action would remain. 



(150.) The members of the series illustrated by the sections visible in this 

 country, near to the line of No. 20, are: — 1. The Lower green-sand; 2. The 

 strata thence to the Portland-stone, — including some doubtful traces of the 

 Hastings-sand, and the Purbeck strata ; 3. The Portland stone itself, with 

 the usual fossils; 4. In a few places, the Portland sand; 5. The Kimmeridge 

 clay. The following details could not be arranged in exact succession, as por- 

 tions of many different groups are frequently found in the same pits ; but the 

 ists are as far as possible disposed in the descending order of the strata, 

 beginning with the Lower green-sand. 



* The top of the church tower at Aylesbury is, according to Mr. Bevan, 425 feet above the sea. 



