Vol. 69.] BATHONIAN HOCKS OF IHE OXFORD DISTRICT. 485"» 



The Combrasli. 



This consists of rubbly non-oolitic limestones of various degrees 

 of coarseness containing the normal Cornbrash fauna. The total! 

 thickness appears to be only about 17 feet, 1 of which the upper 

 part is characterized by Microthyris lagenalis, the lower by 1'ere- 

 hratula intermedia. Cephalopods are far from abundant, there- 

 being apparently but two records : namely, Clydonicevas ^discus 

 from Kirtlington 2 and Macroceplmlites macrocephalus from 

 Witney 3 



The Forest Marble. 



This formation alters considerably when traced from west to 

 east. On the west it consists for the most part of compact shelly 

 and oolitic limestones, with only minor bands of clay or marl; in 

 the centre of the district the false-bedded limestones are broken up 

 into groups by numerous bands of horizontally-bedded marl or 

 clay ; while on the east clays and marls predominate, and the 

 limestones on the whole are but slightly oolitic and very marly. 



The thickness in the area under consideration is only from 21 to- 

 28 feet, while at Alderton, south-west of Malmesbury, the thickness 

 is about 60 feet.' 1 



The incoming of the Forest Marble is represented by a well- 

 marked eroded surface at the top of the Great Oolite, which is often, 

 much ripple-marked and bored and locally covered with oysters ; 

 the lower beds of the Forest Marble also contain fragments of the 

 underlying beds. 



The Great Oolite. 



Except for the uppermost part, which approaches the Forest 

 Marble in lithological character (so much so that in 1860 Prof. 

 Phillips included the top of the Great Oolite in the Forest Marble 5 ; 

 this he corrected in his ' Geology of Oxford,' in 1871), the Great 

 Oolite contrasts remarkably with the overlying Forest Marble, in 

 that the series is almost entirely calcareous, false bedding is absent, 

 and the rocks are as a whole much finer in texture. The eastward 

 thinning of the Great Oolite is very remarkable : although there- 

 are certain beds which are easily recognizable in all the sections, 

 the intervening beds are found to vary considerably, both in thick- 

 ness and in lithological character, and it seems probable that some 

 of them actually die out by a series of overlaps before reaching the 

 east of the district. 



1 H. B. Woodward, ' Geology of the Country round Oxford ' Mem. Geol. 

 Surv. 1908, p. 4. 



2 J. Phillips, 'Geology of Oxford & the Valley of the Thames' 1871, 

 p. 243. 



3 H. B. Woodward, 'Jurassic Eocks of Britain' Mem. Geol. Surv. vol. iv 

 (1894) p. 446. 



1 S. H. Eeynolds & A. Vaughan, Q. J. G. S. vol. lviii (1902) p, 748. 

 s J. Phillips, Q. J. G. S. vol. xvi (1860) p. 118. 



