208 MINERALOGY OF THE SOUTH OP ENGLAND. 



1. Sand and Plastic Clay. Of these two minerals the 

 sand is the most extensive and continuous, and the clay 

 occurs filling up basins and hollows in it. The clay va- 

 ries in colour, being white, gray, yellowish-brown, and 

 red. The white and gray varieties are potters' clay. It 

 sometimes contains beds of brown coal, from one foot to 

 three feet thick ; and beds of ironstone, and ferruginous 

 sand, occur connected with it, and generally lying over 

 it. 



2. London or Blue Clay. The bed which has received 

 this name, is found immediately under the gravelly soil 

 on which London is situated. Of all the strata over the 

 chalk in the south of England, it is of the greatest extent 

 and thickness ; and the number, beauty, and variety of 

 the petrifactions which it contains, render it the most in- 

 teresting, and the most easily distinguishable. It consists 

 generally of a blackish clay, sometimes very tough, and 

 occasionally mixed with green earth and sand, or with 

 calcarious matter. It contains also numerous flat sphe- 

 roidal cotemporaneous nodules of hard marl, or clayey 

 limestone, which lie in regular horizontal layers, at une- 

 qual distances, generally from four to forty feet apart. 

 These nodules are well known by the name of Ludus 

 Helmontii, or Septaria, from their being divided across 

 by partitions or veins of calcarious spar, which are ge- 

 nerally double. In their cavities are frequently found crys- 

 tals of calcarious spar, and of heavy spar. The septaria 

 are surrounded by crusts which contain a smaller propor- 

 tion of carbonate of lime than the central part. They of- 

 ten contain organic remains. 



Besides the clay, marl, sand, and carbonate of lime, of 

 which the main body of this bed consists, several other 

 substances are dispersed through it in smaller quantities. 



