342 JURASSIC, 



be seen in geological collections.^ Its precise horizon appears to be at the top of 

 the chief Building-stones. Many specimens are obtained from ploughed fields 

 north-west of Tisbury. 



The Portland Sands are exposed to the south of Devizes, and 

 further north the Portland Beds occur for the most part in straggling 

 outliers. 



At Swindon the Portland limestone is about eight feet in thick- 

 ness, and contains casts of shells, like the Roach of the Isle of 

 Portland. The sands beneath (25 feet thick) contain irregular beds 

 of hard calcareous sandstone (Swindon stone) ; and another bed with 

 casts of shells, and two layers of hard limestone occur below. It 

 has been stated that evidence of an alternation of marine (Portland) 

 and freshwater (Purbeck) conditions is to be observed at Swindon, 

 but the evidence has been questioned by Prof. J. F. Blake, who 

 points out that the two series are locally unconformable.^ The 

 beds are exposed in quarries to the west of the old town of 

 Swindon, and also at Coate, to the south-east, where the beds 

 of stone have been termed the Coate beds. At Bourton south 

 of Shrivenham, chalky oolite (eight feet) rests on hard blue fossil- 

 iferous limestone, grouped by Sir A. Ramsay with the Portland 

 Sands, ^ and as equivalent to the lower shelly bed at Swindon. 



The beds are developed in Buckinghamshire and Oxfordshire, 

 where the sands have a thickness of 50 to 80 feet.* At Shotover 

 the Portland Sands are overlaid by the Iron Sands of the Lower 

 Greensand. (See sequel.) The stone-beds have been worked near 

 Garsington, Great Hazeley, Thame, Long Crendon, Brill (20 feet), 

 Quainton Hill, and Hartwell near Aylesbury. At Hazeley the 

 stone is about eight feet in thickness, and consists of white lime- 

 stone resting upon grey sandy oolite. The Aylesbury limestone is 

 a marly and shelly limestone, worked at Hartwell and other places 

 south-east of Aylesbury. Aylesbury is situated on Portland Beds, 

 which occur also at Bierton, and at Whitchurch, not far oflf.^ At 

 Hartwell many Ammonites and other fossils have been preserved 

 by Dr. John Lee in the walls that border the squire's park. 



Further north the beds are not developed, unless we include the 

 somewhat doubtful representatives at Speeton,'' which consist of 

 clay containing Portland fossils, but the beds are not well exhibited. 

 A band of coprolites in the clay was formerly worked at New 

 Closes Cliff. 



In the Sub-Wealden boring, sandy beds and sandstone, con- 

 taining chert-nodules, having a thickness of no feet, and com- 

 mencing at a depth of 180 feet, have been classed as Portland 



1 Fitton, T.G.S. {2), iv. 255 ; Etheldred Benett, Catal. of Organic Remains of 

 Wiltshire, 1831, p. iv. 



- See Godwin-Austen, Q. J. vi. 464 ; Morris, Proc. Geol. Assoc, iv. 54S ; and 

 J. F. Blake, Q. J. xxxvi. 203, 209. 



3 Explan. Sheet 34 (Geol. Survey), p. 27. 



* See Fitton, T. G. S. (2), iv. 272. 



* Phillips, Geol. Oxford, pp. 326, 413. 



^ J. Morris, P. Geol. Assoc, iv. 547 ; Judd, Q. J. xxiv. 231, xxvi. 326 ; Strang- 

 ways, Geol. Oolitic Rocks, etc. S. of Scarborough, p. 23. 



