320 Reviens — The Geological Siirvey — 



this generalization might not be fully realized. Within this particular 

 area there is not much difficulty in drawing the line between the 

 attenuated representative of the Kimeridge Clay and the several 

 Portlandian beds above it. But a question which may not trouble the 

 chartographer arises, as to how far these lithological distinctions hold 

 good over all that part of the South of England, from Dorset to the 

 borders of Bedfordshire, where Kimeridge-Portland rocks are developed. 

 If we are to judge by the invariant fossils, it seems probable that 

 a given fauna was in one place being buried in clay, yet elsewhere in 

 sand or even in calcareous deposits. Hence, what is regarded as 

 Kimeridge Clay in the Isle of Purbeck may represent an horizon 

 classed as Portlandian elsewhere. This is too large a question to be 

 discussed within the limits of a review beyond intimating a belief that 

 possibly the complete sequence at Swindon is the most likely place 

 to aiford a clue to the mystery. It is well known that J. F. Blake 

 suggested the idea of a difference in the age of the several groups 

 classed as Portland Stone (Upper Portlandian), those in the north-east 

 being regarded as possibly older than those in the south-west, and 

 particularly in Portland Isle. 



The development of Purbecks within the area is so slight as to call 

 for no comment, but the Ironsands of Shotover, including the Bx-ill 

 and Muswell Hill outliers, ax-e of considerable interest, more especially 

 from the circumstance that these were previously mapped by the 

 Geological Survey as of Lower Greensand age. This was certainly 

 contrary to the opinion of the older geologists, and also of Professor 

 Phillips, who in the "Geology of Oxford," pi. xvi, figures species 

 of Paludina, Unio, and Cyrena from these beds, some of them being 

 new species, but, as usual, withoiit description. There seems to be 

 but little doubt that these Shotover Sands represent the marginal 

 deposits of the great Wealden basin, and are themselves probably on 

 the horizon of the Hastings Beds. By way of accounting for the 

 previous interpretation of the Survey, it is stated that a ferruginated 

 portion of the Portland Stone, containing Trigonia JDamoniana and 

 other marine fossils, had in some way led to the idea that there might 

 be a passage from Portlandian to Ironsand conditions. This view 

 seems to leave the intervening Purbecks out of consideration 

 altogether. Sandy beds south of jS'ewnham possess a scanty fauna of 

 a Lower Greensand character, and these are succeeded in the south- 

 east corner by the Gault. 



The Superficial Deposits of the area are classed under the heading 

 "Pleistocene" and "Recent," but since the writer (Mr. Pocock) 

 regards it as probable that the country has been subjected to subaerial 

 denudation "ever since the Eocene period," there seems to be 

 a possible margin for deposits anterior to the Pleistocene, such for 

 instance as some of the Plateau-gravel, in their memoir called Plateau 

 Drift. Without entering into this subject, a glance at the history 

 of the River-gravels may be of interest. There are four terraces in 

 the Oxford district, of which the highest has not hitherto been found 

 to contain fossil remains. The other three, which together constitute 

 the low-level group, not rising more than 50 feet above the present 

 rivers, contain the bones of Elepltas antiquus, of the mammoth, woolly 



