318 Reviews — Geological Survey of England — Beading. 



{Upper Bagsliot Beds. 

 Bracklesham Beds. 

 Lower Bagshot Beds. 

 London Clay. 

 Beading Beds. 



Cretaceous f ^PP*^"" ^^^^^• 



OBETACEOus | Middle Chalk. 



Only the Middle and Upper Chalk come to the surface in this 

 district, but at Winkfield, in Windsor Forest, a boring passed through 

 the whole formation, and the thickness was found to be 725 feet, 

 of which 219 feet was Lower Chalk, 169 feet Middle Chalk, and 

 337 feet Upper Chalk. 



The Chalk exists throughout the district, but is only found at the 

 surface over a comparatively small part, for, in the southern half of 

 the area and in parts of the northern half, it is covered by Eocene 

 strata often of great thickness, and in other parts the Chalk is hidden 

 under beds of Drift. 



The Middle Chalk is divided into two zones, namely : — 



2. The zone of Terehratidina (gracilis ? var. lata, Eth.). 



1. The zone of Bhynchonella Ctwieri, D'Orb. 



The zone of Terehratulina consists of smooth white Chalk, and in 

 it nodules of flint are occasionally found. It has been termed the 

 zone of Terehratulina gracilis, but it is now known, through the 

 researches of Dr. F. L. Kitchin, that this species does not occur 

 below the uppermost division of the English Chalk. The name to 

 be used for the species of Terehratulina in the Middle Chalk has 

 yet to be decided; it has been called T. gracilis, var. lata, by 

 Mr. Etheridge. 



The Middle Chalk runs down the Thames Valley from Goring 

 and Streatley by Basildon to Pangbourne. Details of the several 

 exposures of this division are given on p. 8 by Mr. Jukes-Browne. 



The Upper Chalk consists of soft white Chalk, more or less evenly 

 bedded, with numerous irregular nodules of flint along the planes of 

 bedding, and sometimes in the Chalk itself between the planes. 

 Thin seams of tabular flint occasionally occur along the bedding, 

 planes, or fill fissures or joints inclined at various angles to them. 

 At its base is the Chalk Bock, a cream-coloured limestone with 

 glauconitic grains and many green-coated nodules. 



The Upper Chalk is divided into several zones, only the three 

 lower of which have been identified in the Reading district, namely : 



3. The zone of Micraster coranguimim, Leske. 



2. The zone of Micraster cortestiidinariiim, Goldf. 

 1. The zone of Holaster planus, Mant. 



(1) The zone oi Holaster planus includes the Chalk Rock, which, 

 as has been said, forms the base of the Upper Chalk. The average 

 thickness of this zone in the Thames Valley is about 20 feet. Several 

 localities and sections are recorded, e.g., an old quarry facing the 

 Thames in Harts Lock Wood opposite Basildon, and in the road- 

 cutting on White Hill, east of Goring. 



(2) The zone of Micraster cortestudinarium, Goldf. The average 

 thickness of this zone in the Thames Valley is about 60 feet, and 



