190 THE STOEY OF THE EARTH AND MAN. 



range of the Jura ; but which the English geologists 

 often name the " Oolitic," from the occurrence in it 

 of beds of Oolite or roe-stone. This rock, of which 

 the beautiful cream-coloured limestone of Bath is an 

 illustration, consists of an infinity of little spheres, 

 like seeds or the roe of a fish. Under the microscope 

 these are seen to present concentric layers, each with 

 a radiating fibrous : structure, and often to have a 

 minute grain of sand or fragment of shell in the 

 centre. They are, in short, miniature concretions, 

 produced by the aggregation of the calcareous matter 

 around centres, by a process of molecular attraction 

 to which fine sediments, and especially those con- 

 taining much lime, are very prone. This style of 

 limestone is very abundant in the Jurassic system, 

 but it is not confined to it. I have seen very perfect 

 Oolites in the Silurian and the Carboniferous. The 

 Jurassic series, as developed in England, may be 

 divided into three triplets or cycles of beds, in the 

 following way : 



fPurbeck Beds. 



Upper Jurassic -j Portland Limestone. 



(Portland Sand. 



( Kimmeridge Clay, etc. 



Middle Jurassic \ Coral Rag, Limestone. 



(Lower Calcareous Grit, Oxford Clay, etc. 



( Cornbrash and Forest Marble. 



Lower Jurassic* ... 4 Great and inferior Oolite, Limestone. 

 (Lias Clays and Limestones. 



These rocks occupy a large space in England, as 



* This last group is very complex, and might perhaps admit of sub 

 division, locally at least, into subordinate cycles. 



