PORTLAND ROCKS OF ENGLAND. 215 



as Pholadomya tumida (rustica, Ph.). They are the strict representa- 

 tives, not as Prof. Phillips seems to have thought, of the " basal 

 sands " of Swindon, hut of 'No. 6 of that station, the great series of 

 sands which lie beneath the fossiliferous zone. This is important, as 

 showing that here, at least, the sands have scarcely diminished in 

 thickness, though, from the order of the beds as given by Dr. Fitton, 

 it is plain that we cannot trace individual portions ; for the sand and 

 clay which should represent jN'os. 4 and 5, seem here reversed in 

 position. 



The whole interpretation, however, of this Oxfordshire district 

 must be postponed till after the description of the remainder of the 

 Portland rocks in Buckinghamshire. 



BuckingJiamshire. 



There is a singular uniformity in the deposits, which spread over 

 a considerable district, here, commencing on the south at a line from 

 Brill to Thame, and ending on the north at a line from Quainton 

 to Stukeley, beyond which no deposits of this age have been traced. 

 Numerous sections in this area are given by Dr. Fitton. Mr. Brodie 

 has given part of that at Brill*; and the succession at Hartwell is 

 figured by Saemannf with much accuracy. A generalized account, 

 therefore, of the beds will be of most value, indicating where each 

 portion may best be seen (see figs. 8, 9, PI. YIIL). 



In some places the top of the Portland rock is uncovered or is fol- 

 lowed by ferruginous sands of a far subsequent date ; but in many 

 the succeeding rocks are white limestones and dark clays with 

 freshwater fossils, usually referred to the Purbeck. In these cases 

 there is no such tumultuous river with large Portland blocks as 

 has carved out the hollows at Swindon ; but the beds follow in an 

 undisturbed manner, as they do in the Isle of Portland and at Chicks- 

 grove, or even more so. In the quarry at Hartwell there is a great 

 erosion of the uj)per beds, and the space is filled with dark stony 

 earth; but this is in the freshwater strata only, and there is no 

 stone above to give a posterior limit to the age of the erosion. At 

 Long Crendon also is a bed immediately above the Portland, which 

 has at its base great pieces of a white calcareous stone imbedded in 

 a greenish white clay ; but these are not Portland stones, and there 

 is no proof of erosion. At South Oving, at Quainton, and at Brill 

 the succession is very regular, and no indication to the contrary is 

 given in any of Dr. Fitton's sections. AYe may conclude, therefore, 

 that there were never any more Portland rocks deposited in this 

 district than those now seen, and that they are not merely dimi- 

 nished in appearance by the overlap of more modern strata. 



No. 1. The uppermost Portland rocks in all the quarries seen are 

 composed of compacted shell-brash — the size and quality of the par- 

 ticles probably depending on the shells broken up, as they vary very 

 much. The absence of oolite in this district is remarkable (what 

 does occur is in the Purbeck at Brill), a point in which these beds 



* In Wright's Monograph of Brit. Foss. Echin. Pal. Soc. p. 354. 

 t De Loriol and Pellat, loc. cit. pi. i. fig. 7, and p. 189. 



