296 MINERALOGY OF THE SOXJTH OF ENGLAND. 



dip is very considerable, being about 45. On the dip of 

 the other sides, no observations have hitherto beenmade 

 The depth of the chalk below the surface at London 

 must be very considerable; since, though wells have been 

 sunk several hundred feet, it has never been reached; but 

 at a few miles south of the metropolis, the chalk is fre= 

 quently come to. 



The formations described by Mr. Webster as lying 

 over the chalk, and in these basins in the south of Eng- 

 land, are the following : 



1. The lowest marine formation over the chalk, inclu- 

 ding the plastic clay, and sand, together with a particular 

 clay, named the London Clay. 



2. The lower fresh water formation, which rests im- 

 mediately on the preceding formation. 



3. The upper marine formation. 



4. The upper fresh water formation* 



5. Alluvium. 



Chalk Formation. The chalk which forms the sides 

 and bottom of the basins, occurs distinctly stratified, and 

 the strata vary in thickness from a few inches to several 

 feet. The whole formation may be considered as com- 

 posed of three great stratified beds, the undermost of 

 which is named chalk marl ; the second hard chalk, with- 

 out flint ; the third or uppermost, soft chalk, with flint. 

 The chalk marl varies in colour, being gray, yellowish, 

 brown : it is softer than true chalk, and on exposure to 

 the weather it rapidly disintegrates. It contains cotem- 

 poraneous nodules, and also beds of a more indurated 

 marl, named gray chalk, from its dark colour. Like all 

 argillaceous limestones, it possesses, in a considerable de- 

 gree, the property of setting under water, when calcined 



