CHAPTER IX. 



JURASSIC PERIOD. 



THE Jurassic strata, comprising the Lias and the Oolitic 

 series, succeed the Triassic marls with complete con- 

 formity, the passage from one system to the other being 

 through a group of grey marls, shales, and limestones, 

 which are known as the Rhsetic or Penarth Beds. The 

 Jurassic system is divisible into three rock groups or 

 series. 



The Lower or Liassic series is essentially a clay forma- 

 tion, with occasional bands of lime stone, sand, and ironstone 

 of variable thickness. 



The Middle Jurassic series consists mainly of limestones, 

 with only subordinate bands of sand and clay. 



The Upper Jurassic, again, is an argillaceous series, the 

 limestones being discontinuous, and sometimes absent or 

 replaced by clays. 



The most persistent formations are the three great clays : 

 the Lower Lias, the Oxford, and the Kimeridge Clays. 

 These range all across England, and form broad tracts of 

 low-lying land ; while the intermediate limestones, where 

 they are well developed, form long ridges, with escarp- 

 ments facing the west or north-west, in consequence of the 

 prevalent easterly dip. 



1. Stratigraphical Evidence. 



Ehcetic and Lias. The Ehaetic Beds, though in Britain 

 they are quite a subordinate division, are of special interest, 



