OXFOED BEWAGE-FARM, AT SAN DFORD-ON-TH AMES. 319 



give the surface of the concretionary stone shown in no. 1. In 

 order to prove this to some extent, heights of 15 feet and 14 feet 

 6 inches were set off above the base of the marl, corresponding to 

 the observed thicknesses in Sandford Hill ; and lines drawn through 

 these points parallel to the assumed bed of rock in the Grit repre- 

 sented the tops of the marl and limestone, if present. This construc- 

 tion showed that instead of the Coralline Oolite being denuded, as 

 was at first supposed when marl was found under the clay of trial- 

 hole no. 3, it had entirely changed its character in the short distance 

 (1 mile) from Sandford. 



Also, as the upper line intersects trial-hole no. 3 very nearly at 

 the depth where marl was found, there can be little doubt as to the 

 general accuracy of the section. 



As coral-bearing strata are found on either side at Headington and 

 Cumnor, there seems to have been here a gap in the reefs, which is 

 filled with a more clayey deposit ; and it is suggested that the influx 

 of clay may have been due to the muddy discharge from some river 

 which might hinder or altogether check the growth of corals, while 

 it was favourable to that of oysters. The width of the marl deposit 

 cannot be more than 3| miles, that being the distance between 

 Headington and Cumnor ; so the supposed river cannot have been 

 very large or very far off. Further, the rapid thinning-out of the Coral- 

 line OoKte of Sandford Hill (within possibly a length of | mile) points 

 to a further constriction in the river's influence, which (if reliable) 

 gives a possible clue to the direction from which it flowed, viz. S.E. 

 It seems therefore probable that the Palaeozoic rocks known to exist 

 beneath London may have been above water in the later Oolite 

 period and have had considerable extension to the S.W. and E. 



A considerable portion of the sewage-farm is upon the Calca- 

 reous Grit ; and the many trenches have yielded the following few 

 fossils : — 



Ammonites plicatilis. Ostrea gregaria. 



cordatus (?). , sp. (small). 



Modiola bipartita. Serpula, sp. 



GerviUia ayiculoides. Sponge ? 



Pecten vagans. 



Pema, sp, Hemipedina marchamensis, from Hey- 

 Ostrea dilatata (large). ford Hill Lane, Littlemore. 



Many beds of rock were met with in the sand ; but no continuous 

 section was obtainable. The lower beds seemed hard and flaggy with 

 oolitic granules on the surfaces ; those nearer the top frequently 

 presented irregular sponge-like shapes, and were very soft and fri- 

 able ; while the upper beds were generally concretionary and more 

 compact. 



The bottoms of the small vaUeys which intersect the farm are 

 filled with peat from 4 to 6 feet deep, in which have been found 

 Eoman pottery (at 3 feet), freshwater shells, bones of deer, domesti- 

 cated pig, cow, horse, and dog. Under the peat, patches of gravel 

 of a very local character are found, quite unlike that in the Thames 

 valley. 



