INFERIOR OOLITE AND LIAS NEAR TORTWORTH. 



449 



will first, therefore, give a short sketch of the overlying formations, showing where 

 they pass into each other, and where they are obscured or broken ; and then point out 

 the range and distribution of the Silurian deposits, concluding with a few words upon 

 the effects due to the intrusion of the trap rocks. 



Inferior Oolite. 



The district is flanked upon the east by a succession of bold tabular promontories, the most 

 prominent being the trigonometrical station at Stinchcombe, north of Wotton-under-Edge, and 

 having a height of 800 to 900 feet above the level of the sea. The summits of these hills consist 

 of the inferior oolite, which constitutes, as before stated, the escarpment ranging thence to the 

 eastward of Cheltenham, constituting the chief mass of the Cotteswold Hills, the strata dipping very 

 slightly to the east and south-east, p. 14. The well-known characters of the various beds con- 

 stituting this formation must be sought in other works. The Avon, the chief of the little streams 

 which water the Tortworth tract, rises at the foot of one of these oolite hills to the south of Wotton- 

 under-Edge. 



Lias. 



This great formation which constitutes the base of the whole oolitic system extends over a great 

 part of the plain and undulating country between the older rocks of Berkeley and Tortworth and 

 the inferior oolite of the hills. Through the labours of Mr. Lonsdale, who has laid down the course 

 and outline of this and the overlying oolitic formations in Gloucestershire, we first learned that it 

 consists here of two subdivisions, the marlstone and lower lias. The first of these is chiefly made 

 up of yellow and ochraceous sandstone, passing into blue calcareous grits, and charged with abundance 

 of GryphcBa gigantea, Pecten cequivalvis, and other fossils. It rises, for the most part, to a height 

 of several hundred feet on the sides of the oolite hills, and is separated from the inferior oolite by 

 only a very narrow zone of upper lias shale. This marlstone is exposed through all the deep 

 recesses which penetrate the oolite hills, and advances at some points far into the Vale of Glou- 

 cester. (See Map.) Thus at Newnham windmill, situated on one of the sloping terraces below the 

 oolite of Stinchcombe Hill, and overlooking the lower lias of the vale of the Severn, it is largely 

 quarried as a road stone and transported to distant parts of the Berkeley district. In the older 

 classifications of British rocks, this deposit (here, perhaps, not less than 100 feet thick) is consi- 

 dered to be the base of the inferior oolite, a place in which it can no longer be recognized, since 

 not only has it a community of fossil character with the other parts of the lias formation, but it 

 is also overlaid by an upper lias shale or marl, which completely separates it from the inferior oolite. 

 (See p. 16.) The lower lias, in its usual forms, spreads out from beneath the terraces of marlstone, 

 and extends to the edge of the older rocks, in some places exhibiting towards its base, a conform- 

 able passage into the New lied Sandstone, in others abutting or resting abruptly on Old Red 

 Sandstone and Silurian rocks. Passages from the lias into the underlying New Red Sandstone can 

 be observed in the sides of Whitcliffe Park Hills, south of Berkeley ; but the clearest and best in- 

 stances are to the east of Wick war, near Sturt Bridge, on the sides of the new road ascending to 

 Wotton. In the same neighbourhood I have noticed at several points the well-known bone bed, 

 subordinate to the lowest layers of black shivery shale of this formation, and immediately sur- 



