Mr. Webster on the Strata lying over the Chalk. 187 



In the Isle of Wight basin, both in Dorsetshire and Alum bay, 

 beds of Iron stone and ferruginous sand occur connected with this 

 clay, and generally lying over it. Considerable rocks of it are seen 

 about Studland, and the Druidical monument, called the Agglestone ; 

 near that place is a huge block of this bed. 



A stratum of sand, containing green particles, frequently occurs 

 near the chalk. It is seen in Alum bay without fossils ; at Read- 

 ing it is found containing oyster shells. This green sand is easily 

 distinguished from that below the chalk, as it is never indurated. 



2. London or Blue Clay. The stratum which has received this de- 

 denomination is found immediately under the gravelly soil on which 

 the metropolis is situated. Of all the strata over the chalk in this 

 country, it is of the greatest extent and thickness : and the number, 

 beauty, and variety of the organic fossils which it contains, renders 

 it the most interesting and the most easily recognizable. 



It consists generally of a blackish clay, sometimes very tough, at 

 other places mixed with green earth and sand, or with calcareous 

 matter. 



It contains also numerous flat spheroidal nodules of indurated 

 marl, or argillaceous limestone, which lie in regular horizontal 

 layers, at unequal distances, generally from four to ten 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 calcareous spar, which are generally double. In their ca- 

 vities are frequently found crystals of calcareous spar and of sulphat 

 of barytes. The septaria are surrounded by crusts which contain 

 a smaller proportion of carbonat of lime than the central part. They 

 often include organic remains. 



Besides the clay, marl, sand, and carbonat of lime, of which the 

 main body of this stratum consists, several other substances are dis- 



2 A 2 



