lxxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 1913, 



This, combined with a number of well-sections near the outcrop, 

 rendered possible the tracing of the line along which the base of tbe 

 Gault lies at sea-level, that is the contour-line marked on the map. 

 Next, the deep borings in which the base of the Gault has been 

 identified provided a series of points of precision. There were also 

 available a large number of well-sections which, without reaching 

 the base of the Gault, gave the level of the base of the Chalk or in 

 other ways provided materials for an estimate. Lastly, when all 

 other evidence failed, I used the numerous well-sections which 

 have proved the level of the top of the Chalk under Tertiary beds, 

 and estimated the thickness of the Upper Cretaceous rocks from a 

 consideration of the nearest deep boreholes or measured sections. 

 These last estimates are open to the objection that the Tertiary 

 beds cut across the Chalk, in places markedly so. In the western 

 and north-western parts of the area this method could only be 

 used with caution. For the most part, however, the results 

 obtained were so consistent that my confidence in them was 

 somewhat restored. 



Altogether seven contour-lines, ranging from 500 feet above the 

 sea (shown on the map as +500) to 2500 feet below the sea 

 (shown as —2500) have been traced. Of these the -f 500 line 

 is evidenced in a few places only, where a neighbouring outcrop 

 either attains or closely approximates to that height, as at Leighton 

 Buzzard, near Bletchley, in the Yale of Wardour, north-west of 

 Penshurst, in the Weald, in the Isle of Wight, and in Dorset and 

 Devon. The —1000 line has been drawn, so far as regards the 

 London Basin and the ground north of it, mainly by reference to 

 deep borings. All lines below —1000 are confined to the Hamp- 

 shire Basin. They have been drawn by reference to the level of 

 the top of the Chalk as found in borings, and by consideration 

 of the thickness of the Upper Cretaceous rocks as developed in the 

 nearest outcrops. 



The map thus constructed gives a comprehensive view of the 

 various flexures under consideration. It illustrates, in the first 

 place the increase in intensity and frequency of the folding towards 

 the south, and shows how in each syncline the axis hugs the 

 southern margin. You may notice, for example, the broad spacing 

 of the 0, —500, and —1000 lines on the northern side of the 

 London Basin, and the comparative crowding of the same lines ou 

 the southern side. The two sides of the Hampshire Basin not only 

 show the same difference, but the greater depth of the syncline as 

 compared with that of the London Basin is well brought out. 



