44 THE STORY OF THE EARTH. 



tricts; and the red soil on limestones. Beds of 

 volcanic ash, exposed upon the surface, may be- 

 come broken up by atmospheric decomposition 

 and converted into clay. 



The bedding Of clays is frequently marked by 

 the occurrence of thin layers of earthy limestone, 

 or of concretions termed septana, which are dis- 

 tributed in parallel layers, on zones where car- 

 bonate of lime was abundant. These concretions 

 probably mark near approach to the limit to which 

 the ancient mud was carried in the sea, where the 

 Sediment was becoming replaced on the ocean 

 Boor by calcareous layers of organic Origin. The 



septaria often contain upwards of 50 per cent, of 



Carbonate of lime, and their occurrence app< 

 t<> mark, either oscillations in level of the sea bed 

 which varied the distance to which sediment was 

 :ed, or indicates that the land area which was 

 being worn away to form the new deposit became 



more calcareous. 



of the most interesting clay deposits in 



l gland is the inflammable claw known as kim- 



lay, found in Lincolnshire and Cambrid 



«•;; as on the D01 ast, Some 



lay when distilled yield as much as 



40 per cent, of paraffin, naphtha, tar and Ik 



. which are similar to products of COal tar. 



I < < hemical Substances derived from coal bc- 



Igin, it is probable that the in- 

 flammable < ha rad cr of the clay is due to the -row th 



aarine algae, though no tracesof plant remains 



found anion- the remains of marine shells 



whit h (row d the deposit. 



Almost all 1 I Id the minerals, iron pyrites, 



i lueli are I losely dependent upon 



a other. Iron pyrites mineral ils, and 



