GEOLOGY 



1895, v. 5,40, etc.), which work has been largely used in the description 

 of the succeeding Jurassic formations. 



CORALLIAN 



The Corallian forms a very well marked band running across the 

 county from the Cole to the Thames. Below it is a thick bed of clay 

 the Oxford Clay already described and above it there is another thick 

 bed of clay the Kimeridge Clay ; and the Corallian, essentially a 

 calcareous formation with hard limestones, rises above these two clay 

 beds as a ridge of elevated ground. Shrivenham, Coleshill, Faringdon, 

 Stanford, Kingstone Bagpuize, Garford and Cumnor are situated on it. 

 In the north near Wytham there is a small outlying patch of Corallian 

 rocks which reach a height of 583 feet above sea level. The land is 

 largely laid out in cornfields. The thickness of the formation in Berk- 

 shire is from 50 to 80 feet, but it is very variable both in thickness and 

 in character, and though, as has been said, it is essentially a calcareous 

 formation, the lower part is often sandy or clayey and in places sands 

 and even clays occur in the upper part. 



The formation is highly fossiliferous and has been divided into two 

 fossil zones, the lower of which is known as the zone of Ammonites 

 perarmatus and the upper as that of Ammonites p/icati/is, and both these 

 ammonites are found in Berkshire. 



The Lower Corallian was found to be 35 feet 3 inches thick in a 

 boring at Shillingford north of Wallingford. It consisted to a con- 

 siderable extent of clays and partly of sand, but both the clays and sands 

 contained bands and layers of stone. 



At Marcham the sands are current bedded and in places ripple 

 marked, and Mr. H. B. Woodward remarks that their irregular cemen- 

 tation into doggers and into bands of sandstone was well shown in the 

 quarries there. These sands are fairly fossiliferous, and Marcham is 

 famous owing to the fine examples of Ammonites perramatus, mostly in 

 the form of casts, which have been found there. Pebbles of quartz, 

 lydite, etc., frequently occur in the Lower Corallian. 



The Upper Corallian consists mainly of limestone, oolitic, pisolitic 

 and shelly beds, with very subordinate sands and clays and with rubbly 

 coral rag near the top. Its thickness was 44! feet in the Shillingford 

 boring. At Shrivenham Professor Hull noted a local deposit of ferru- 

 ginous sands separated from the coral rag by a parting of clay, and Mr. 

 Woodward remarks that it may be difficult in some places, in the 

 absence of the upper coral band, to discriminate between this deposit and 

 the Lower Greensand, which now and then rests on the Corallian. 



The Upper Corallian is exceedingly fossiliferous, and one bed, 

 which is termed the Trigonia Bed owing to the abundance of the shell 

 Trigonia perlata, is clearly marked at Faringdon, Fyfield, Marcham and 

 other places. The bed contains the shell Ammonites plicatilis, which gives 

 its name to the fossil zone, and also Ammonites cordatus^ Belemnites, Lima, 

 Ostrea, Ecbinobrissus, Pygaster, etc. 



5 



