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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 21, 



General considerations respecting the distribution of land and 

 water in the English Tertiary area at the commencement of the 

 Eocene period. — In no part of the tertiary area of the south of 

 England is there any indication of a passage either in mineral struc- 

 ture or in organic remains between the Chalk and the Tertiary seizes. 

 With regard to the physical conditions, the change is most marked, 

 and abrupt. Extensive and deep wear of the chalk evidently took 

 place before the commencement of the lowest Eocene deposits. In the 

 neighbourhood of Calais, at St. Vallery-sur-Somme, Pegwell Bay, 

 Upnor, Woolwich, Stortford, and thence to Reading, Newbury, Salis- 

 bury, Newhaven, and Alum Bay, the chalk invariably presents a worn 

 though not very irregular surface, and is strewed over, as before men- 

 tioned, with those peculiar green-coated flints. This mass of flints, 

 although generally not above 1 to 2 feet thick, in itself indicates a 

 wide destruction of the chalk. But independently of this, another 

 denudation had probably previously worn down a very large mass of 

 the chalk along the southern boundary of the London tertiary area. 

 In this direction all the upper beds, and a large portion of the middle 

 beds of the chalk, have been removed ; and it is probably owing to 

 this cause that the chalk, which has been ascertained to be above 

 1000 feet thick at Saffron Walden, and is apparently about 800 to 

 900 feet thick at Luton and along its northern line of escarpment, 

 becomes apparently gradually thinner as it ranges towards London, 

 and eventually becomes reduced at the edge of the escarpment over- 

 looking the Weald to a thickness not exceeding on the average 400 

 feet *. The following diagram (fig. 6), which gives the general re- 

 presentative section, independently of the exact conformation of the 

 surface, from Saffron Walden to the chalk-escarpment above God- 

 stone, exhibits the structure of the chalk here referred to : — 



Fig. 6. — Diagram to illustrate the thinning out of the Chalk in the 

 direction of the axis of the Weald. 



A. Relative position of Saffron Walden. 



B. Chalk escarpment above Godstone, sur- 



mounted with a patch of the Lower Ter- 

 tiary beds. 



a. London Clay. 



b. Lower Tertiaries. 



e. Chalk, 



d. Upper Greensand. 



e. Gault. 



/. Lower Greensand and Wealden. 



X. Point at which the present upper and under sur- 

 faces of the Chafk, if they were prolonged, 

 would converge. « 



* At Bushey near Watford the chalk-marl was reached at a depth of less than 

 400 feet beneath the tertiaries, and at London the lower chalk without flints com- 

 mences at about 250 feet below the surface of the chalk, which is here, as well 



