lxxviii PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OO, 



is taken by the Red Chalk. It is noteworthy that at Holkham 

 both formations occur, and that this is the only case of the 

 occurrence of Red Chalk over Gault. 



We may take it, therefore, as almost proved that while the 

 Gault clay is continuous under the London Basin, except in the 

 north-western corner of Norfolk, there is a general but irregular 

 thinning eastward from Winkfield, accompanied by a northerly 

 thinning, from the outcrop in East Surrey and in Kent. It is to 

 be noted, too, that this is also the case with the Selbornian 

 as a whole, but more decidedly, as the Upper Greensand is absent 

 eastward and north-eastward. 



The Lower Greensand. 



Putting out of consideration the great number of borings in the 

 neighbourhood of Cambridge (made for the purpose of obtaining 

 water from the Lower Greensand), almost all of which are either 

 on the Chalk Marl or on the Gault, and sundry other borings also 

 near the outcrop in various parts, mostly, indeed, on the Gault, 

 few borings reach this formation and eight or nine only pierce it 

 from top to bottom. Nevertheless, what is learnt of it is of more 

 interest than what has been learnt of the Gault, for in these matters 

 change is more charming than constancy, and the underground 

 thinning of the Lower Greensand is a question that has been often 

 before geologists. 



There is sometimes evidence of great thinning at or near the 

 outcrop : thus a boring at Shillingford, near Wallingford, after 

 piercing 144 feet of Gault, proved a thickness of only 25 feet of 

 Lower Greensand, passing then into Kimeridge Clay and thereafter 

 through Corallian beds to Oxford Clay. 



The presence of this formation at the bottom of the Winkfield 

 boring leads one to infer that this occurrence of Lower Greensand 

 deep underground will be found to continue to the southern out- 

 crop, where the formation shows in force ; but there is no evidence 

 how far northward of Winkfield this continuity may reach. It 

 may be to the outcrops to the north-west or to the north. It may 

 be, on the other hand, that there is a thinning out, as along the 

 northern boundary of the Gault, from west-north-west to north. 



Eastward at Richmond there is a thickness of only 10 feet, 

 although at the outcrop, some 15 miles southward, there is a goodly 

 mass. Little farther north probably no Lower Greensand would 

 be found, for in other borings in or near London, from Streatham 



