464 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 19, 



the surface which it occupies over the tract which intervenes between 

 Bourton and the escarpment of the chalk : it is an isolated mass, from 

 about which the upper beds of the Kimmeridge clay have been par- 

 tially removed on every side ; nor do any like beds occur at the base 

 of the cretaceous series, where the gault rests directly on Eammeridge 

 clay. 



§ 2. Swindon. 



On the following morning we commenced with the great quarry 

 west of the town. The mass dips away at a small angle east and 

 south, so that the lowest beds are exhibited on the west and north. 

 At the north-eastern corner of the quarry the succession from below 

 upwards is as follows : — 



1 . Thin seam of black pebbles, of which a portion is always to be 



seen attached to the lowest masses of sandstone. 



2. Stratified sandstone, thin-bedded, pale blue : 5 feet. 



3. Clay, containing a bed of Ostrea falcata, Sow., Perna quadrata. 



Sow. : 1-2 feet. 



4. Sands and sandstones, 25 feet ; the lowest portion most fossi- 



liferous. Cardiwn dissimile, Sow. 



5. Calcareous sand, passing up into pure limestone, 5 feet ; contain- 



ing a bed about 1 foot thick almost entirely composed of Tere- 

 hra Portlandica, Sow. 



6. Above this marine limestone, but not separated by any line more 



distinct than is usual in stratified limestone masses, is a limestone 

 offering some slight differences in appearance and fracture, but 

 containing apparently only freshwater forms. Above this are 

 bands of clays and calcareous sandstones. The whole constituting 

 the freshwater portion of the series. 



7. Thin-bedded calcareous sandstones, with marine shells, in a mass 



of sand, forming the highest portion of the Portland series. 



Above the surface of the beds of Swindon Hill is a considerable 

 capping of drifted materials, resting on an uneven surface, containing 

 materials of various geological ages, and in no way connected with 

 the history of the beds below. 



The geological phsenomena which this quarry presents are of ex- 

 ceeding interest, and it will be perhaps the most intelligible course 

 we can adopt, in order to make them appreciable, if, instead of a dry 

 mineralogical account of admeasurements, we explain briefly the suc- 

 cessive conditions which are indicated at this spot, over an area of 

 fifteen acres. 1st. At a place in the oolitic sea, where the deep-sea 

 mud-bed of the Kimmeridge clay was being deposited, there is evidence 

 of a sudden increase in the; moving power of the water, of the dis- 

 turbance of the materials of the zone of subangular pebbles, and of 

 their outward dispersion. The deposits which immediately follow 

 are of fine clean siliceous sand, or at times with a slight proportion of 

 calcareous matter : the change here indicated is that of a diminution 

 of depth at this spot ; or, what will describe the change more accu- 

 rately (the sea-level being supposed uniform), a rise proceeding from 

 the land-side of this portion of the oolitic sea. 



