Ixxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May IC;OO y 



find it absent at the surface over considerable parts of the bordering 

 Cretaceous tract. The Gault, on the other hand, is a practically 

 persistent mass of clay, and the only part where this clay does not 

 occur at the surface (where lower beds crop out from beneath the 

 Chalk of the London Basin) is far north, in North-western Norfolk, 

 and there only the thin Red Chalk comes between the White Chalk 

 and the Carstone of the Lower Greensand. 



The Lower Greensand, in general a fairly thick formation, has 

 a continuous outcrop round the district of the Weald, except 

 perhaps in the far east of Sussex, where it is certainly thin and 

 apparently sometimes absent. On the northern side of the London 

 Basin it is occasionally wanting at the surface, owing to overlap of 

 the Gault. 



The Wealden beds, from their freshwater and estuarine nature y 

 are more local, and though very thick on the south, in the tract 

 from which the name is taken, are almost confined to that part, the 

 occurrence on the north being insignificant. No one, therefore, 

 expected to find their underground range very great. 



The Chalk. 



The great number of wells reaching the Chalk through Tertiary 

 beds has been noticed, and many of these go to a considerable 

 depth in the Chalk ; but it is rather rare for a boring that starts in 

 the Tertiary beds to pass through the Upper Chalk (the thickest 

 division) into the Middle Chalk, and it is still more rare to find 

 a boring that reaches down from the top of the Chalk to its base. 

 Prestwich was the first to record any such borings, those at Kentish 

 Town and at Harwich, and for many years these alone held the 

 field. 



In the great tract of the London Basin there are only fifteen 

 places where such borings have been made ; but at eight others 

 nearly the whole thickness of the Chalk has been pierced, the 

 sites being either no great distance from where the Tertiary beds 

 come on, or so high on the Chalk that no great thickness of it can 

 have been removed. Besides these others, beginning somewhat lower 

 down in the Chalk, have yet pierced a considerable thickness of it. 



The first new fact that we learn from these deep borings 

 is the thickness of the Chalk ; and this at its lowest is over 

 620 feet (at Streatham), while in the highest case it reaches to 

 more than 1140 feet; and this latter is at Norwich, where the 



