TF. WJiitaker — Chalk of the London Basin. 361 



in some parts of Norfolk, may be 1200 feet or more. This figure, 

 however, is exceptional, the nearest to it recorded beino^ 890 at 

 Harwich [this should probably be 895, the Glauconitic basal 

 bed having been called Upper Greensand]. At East Horsley, in 

 Surrey, a thickness of 817 feet has been proved, but generally 

 speaking we find less, and over a large area 700 feet is not reached. 

 It seems that the Chalk thins toward London (where 650 feet may 

 be taken as about the usual thickness), not only from north and 

 south but also from east and west. [It may be well to add the 

 thicknesses proved by other borings outside London. These are as 

 follows, where the borings have passed through the whole formation, 

 from the overlying Tertiary beds to the underlying Upper Green- 

 sand or Gault :— Loughton 651 feet, Kichmond 670, Cheshunt 681, 

 Chatham and the neighbourhood 680-684, and Winkfield, near 

 "Windsor, 725. In sundry places where the topmost beds are missing 

 we have the following : — Bushey (Herts) 686, Chartham (Kent) 735, 

 and Coombs (Suffolk) 817, which last-mentioned should probably be 

 over 820, for the same reason as given above, for increase at Harwich.] 



2. Divisions of the Chalk. 



Of old the Chalk was usually divided simply into Upper and 

 Lower, the former being also known as the Chalk with flints [but 

 the occurrence of flints is not always a safe guide]. Of late years, 

 however, a further division has been made by the splitting up of 

 the old Lower Chalk into Middle and Lower. This last, too, is 

 often again divisible, by the separation of its lower member, the 

 Chalk Marl. 



Besides these divisions certain zones have been marked off, but 

 chiefly from the tnore or less common occurrence of certain fossils 

 in them. These zones, however, need not be noticed now; it is 

 enough to note the divisions that are based on structural features 

 and that occur over large tracts. 



The Upper Chalk of this great district is marked bj' the general 

 prevalence of flints, mostly as separate nodules, in laj'ers or scattered; 

 but also in continuous sheets. Often, however, we find a considerable 

 thickness in which flints are very rare. No other peculiar structure 

 is notable in this division ; the beds are sometimes thick, sometimes 

 comparatively thin, and usually there are more or less marked planes 

 of jointing. 



The Middle Chalk is, on the whole, without flints ; but sometimes 

 a few flints are found in it toward the middle part. It is thick- 

 bedded and jointed. Its top is generally shown by the presence of 

 hard beds, consisting of cream-coloured crystalline limestone where 

 well developed, and with green-coated nodules. This is known as 

 the Chalk Rock, and is often to be seen in Hants, Wilts, Berks, 

 Oxon, Bucks, Beds, and Herts. It is less clear in Essex and 

 Cambridgeshire, has not been noticed in Suffolk, and but one section 

 is known in Norfolk. In Surrey and Kent, though no bed with the 

 distinct lithological character of the Chalk Rock has been seen, yet 

 microscopic investigation proves that it is represented. The bottom 



