198 NEOZOIC TIME. [CHAP. XI. 



any widespread unconformity, producing rather what may 

 be called a physical and palseontological hiatus, which is 

 intensified by the contrast between the characters of the 

 Chalk and the London Tertiaries. There is, however, a 

 certain amount of unconformity, arising probably from 

 unequal uplift and erosion during the unrepresented time, 

 for, as Dr. Ch. Barrois has pointed out, 1 " the centre of the 

 Tertiary basin of London does not correspond with the 

 centre of the Cretaceous basin of London. Norfolk, the 

 deepest part of the Cretaceous sea, would still have been a 

 depressed region after the Cretaceous if an upheaval of 

 this area or a greater depression of the southern part of 

 the basin had not modified this state of things before the 

 deposition of the Thanet Beds." 



A consideration of the variations in the thickness of the 

 Chalk, as proved by deep borings in the London basin, 

 shows that the Chalk is thinnest beneath or near London, 

 and that it thickens in every direction from that centre, as 

 if the original uplift had produced a low and broad dome 

 which had its centre on the Surrey side of the Thames, 

 and was quite independent of the subsequent upheaval 

 which produced the axis of the Weald. Thus the Chalk 

 is thinnest at Streatham (623 feet) ; beneath London, and 

 thence northward as far as Loughton, it varies between 645 

 and 656 feet ; further north it thickens to 680 at Cheshunt, 

 and north-eastward to over 700 feet at Wickham Bishop 

 and to 890 at Harwich ; from London eastward it thickens 

 to 682 at Chatham, and is over 734 at Chartham, near 

 Canterbury ; westward it is rather thicker at Richmond 

 (671 feet) than below London, and at Bushey, to the north- 

 west, it is about 700 feet. 2 South-west, at East Horsley, be- 

 tween Leatherhead and G-uildford, it is 817 feet, and on the 



1 " Recherches sur le Terr. Cret. Sup.," 1876, p. 179. 



2 This thickness is obtained by adding the difference between the 

 level of the ground at the well and the height of the Tertiary outcrop 



