466 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 19, 



tion. The higlier portions of this part of the series consist of bands 

 of clay, with carbonaceous matter and occasional partings of sandy 

 and calcareous materials, now passed to the state of loose rubbly 

 stone. For a considerable thickness the beds indicate times of tran- 

 quil deposition, of the occasional disturbances of such surface, of no 

 great depth of water, and the proximity of some area of land. 



If the conditions recorded in the Swindon quarry are to be de- 

 rived from its western side alone, it would seem as if this fragmentary 

 and disturbed portion constituted the close of the series of changes, 

 which followed in the long uniform period indicated by the Kimme- 

 ridge clay. The summit of Swindon Hill forms a tabular surface, 

 and owing to the inclination of the beds, the denudation which has 

 taken place at some period long subsequent, has proceeded to a lower 

 portion of the series on the west to what it has on the east ; and 

 (with the exception of a capping of detritus, containing materials of 

 all ages up to the grey-wether-sandstones) the fragmentary and dis- 

 turbed beds form the highest part of the quarry ; the like holds good 

 along the north side of the quarry and part of the eastern, but with 

 the dip of the beds south, these disturbed bands of clays and sands 

 are seen to be surmounted by layers of tranquilly deposited sand- 

 stones in thin layers interstratified with sands, and in these the forms 

 of the marine Portland reappear*. 



We next examined a quarry about a mile from the town of Swin- 

 don, on the north of the road to Coate : the beds in which it is 

 opened belong to the upper calcareous portion of the Portland, and 

 present a like order in the occurrence of the fossil forms with that 

 already described. The limestone mass seems to be surmounted 

 with thin-bedded sandstone. About a quarter of a mile on the same 

 road, but on the south side, is another large quarry : the beds here 

 dip to the north, which would take them beneath those of the quarry 

 just noticed. The beds are sands and sandstones, which agree very 

 exactly with those of the lower portion of the great quarry close to 

 the town. At Broom Hall the lowest sands and sandstones are seen 

 resting on the Kimmeridge clay, with a band of black subangular 

 pebbles at the line of junction, and incorporated with the lower 

 layers of stone. The last place in this direction at which the Port- 

 land beds are worked is on the east side of the reservoir : here also 

 they belong to the lowest part of the series, resting on Kimmeridge 

 clay, and containing the like admixture of lydian-stone pebbles. 

 The ferruginous sands which surmount the compact sandstones, on 

 the level of Day-house Farm, contain beautifally preserved specimens 

 of Trigonia gibbosa, Sow. The thin Kimmeridge clay, which emerges 

 from beneath this mass of Portland, is continued to the rise of the 



* Mr. Brodie has given an account of what he designates as the " Purheck 

 strata " at Svsdndon (Geol. Journ. vol. iii. p. 53). His section is taken at the 

 western end of the quarry, and represents a thickness of 13 feet. The description 

 given by M. Cornuel of the upper Jurassic strata of Vassy agrees well with the 

 series of conditions to be observed at Swindon, the " Oolite vacuolaire " repre- 

 senting mineralogically as well as in its Cyrence, Mytili, and Melanice, the fluvio- 

 marine strata of the top of the Portland. 



