168 NEOZOIC TIME. [CHAP. X. 



these beds, as the rest of the pebbles have come from the 

 neighbouring Jurassic rocks, namely, the Coral Rag and 

 Kimeridge Clay. It does not seem likely that the pebbles 

 of Palaeozoic rocks were derived directly from such rocks, 

 because the localities are so far distant from any outcrops 

 of Palaeozoic rock that are likely to have been exposed at 

 that time, unless they came from a buried continuation of 

 the Mendip range ; neither is it easy to see how they can 

 have been derived from the Trias, for the strata containing 

 them are shore-beds formed in close proximity to the 

 actual land margin. 



These pebble beds extend as far as Baldon, south of 

 Oxford, but near G-arsington and Shotover a very different 

 set of beds come in, the characters of which more resemble 

 those of the Walpen Sands at Punfield ; they consist of 

 variously coloured sands and clays, with beds of iron-ochre 

 and fuller's earth, and they contain freshwater shells 

 (Gyrena, Unio, and Paludina), with pieces of coniferous 

 wood. These beds extend in outliers by Brill and Quain- 

 ton to Whitchurch ; and south of this line, near Thame 

 and Hartwell, there are other beds in which marine fossils 

 have been found. The actual relations of these two sets of 

 beds have not yet been ascertained, but it may be remarked 

 that no intercalation of beds with marine and freshwater 

 fossils has yet been observed, and that the marine beds are 

 generally in close proximity to the Grault. It is possible, 

 therefore, that the freshwater beds are lacustrine and 

 lagoon deposits of a slightly earlier date, and were formed 

 in a low-lying tract of land which was afterwards invaded 

 by the sea. This supposition finds some confirmation in 

 Professor Morris's observation, that at the base of the 

 marine sands near Hartwell there are derived blocks of 

 brown sandstone containing freshwater fossils. 1 



1 See " Geol. Mag.," vol. iv. p. 458. 



