POETIAIJD HOCKS OF ENGLAND. 203 



absence, however, of any intervening Kimmeridge Clay, it can 

 scarcely be so old an episode as the Boulognian. 



Devizes. 



The exposure of Portland rocks in this neighbourhood, as marked 

 by the Geological Survey, is a feeble one. It may, however, be 

 considered as modelled on the Swindon type, though exposing only 

 the lower beds. The topmost part of the Kimmeridge Clay is very 

 sandy, and contains the usual Swindon and Hartwell fossils ; the 

 greyish-yellow sands overlie this ; and above these, again, at Crook- 

 wood, are some fissile limestones, with Pecten lamellosus and Trigonia 

 (jibhosa. 



Swindon. 



The sections in this neighbourhood are the most extensive and 

 interesting out of the islands of Portland and Purbeck; and yet, 

 though the locality has been visited again and again, they really 

 seem almost unknown, and the interpretations placed on the phe- 

 nomena visible in the great quarries themselves are strangely con- 

 tradictory. 



The first point of interest is the relation of the Purbeck to the 

 Portland. Dr. Pitton merely notices the presence of botryoidal 

 limestone over the Portland. Mr. Brodie * describes the Purbeck 

 beds only, proving their freshwater character by the contained 

 fossils, and states most truly that " the surface of the Portland 

 strata has been greatly denuded previously to the deposition of the 

 overljing group ; for in many cases the latter is deposited in hollows 

 and cavities, where the Portland Sand has suffered erosion by water." 

 On the other hand, Mr. Godwin- Austen t states that " with the dip 

 of the beds south these disturbed bands of clays and sands are seen 

 to be surmounted by layers of tranquilly deposited sandstones in 

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

 marine Portland reappear." No such sands and sandstones are 

 recorded by any other observer ; their precise supposed position is 

 not indicated ; the names of the marine shells are not given ; and I 

 have searched for any such in vain. On this statement the author 

 founds the belief that " the Wealden (Purbeck) is not, as has hitherto 

 been represented, a freshwater accumulation of an area of dry land 

 subsequent to the oolitic period, but was contemporaneous with the 

 Portland, and perhaps even with older portions of the oolitic series." 

 This view is not confirmed by the officers of the Geological Survey 

 in their memoir on this district ; but in the report of the excursion 

 of the Geologists' Association to Swindon in 1876 under the guidance 

 of Mr. Moore X it is stated that above a " so-called Purbeck bed," 

 " the regular Portland limestone comes on again rich in casts of the 

 usual Portlandian shells, showing most clearly how ' Purbeck ' and 

 ' Portland ' conditions, inosculated at this spot, and that .... we 

 actually have the Portlandian overlying the Purbeck on the east 



* Quart. Joum. Geol. Soc. vol. iii. p. 53. t Ibid. vol. yi. p. 466. 



X Proc. Geol. Assoc, vol. iv. p. 548, 



f 2 



