JURASSIC. 



253 



JURASSIC. 



Fig. 41.— Section at Whatley, near Frome. (C. Moore.) 



a. Carboniferous Limestone. b. Middle Lias. c. Rubble of Inferior Oolite. 



The Jurassic system includes the several members of both 

 Lias and Oolites, and derives its name from the development 

 of these strata in the Jura mountains between France and 

 Switzerland. The name Jurassic was given in 1829, by 

 Alexandre Brongniart, but the term Jura limestone has been 

 in use from time immemorial.^ 



The Liassic and Oolitic rocks have been divided into 

 groups and stages, which are, as a rule, distinguished by 

 marked lithological features, and by certain assemblages of 

 organic remains. Although some of the chief divisions are 

 remarkably persistent over large areas, yet the minor groups 

 display a considerable variation in thickness when traced 

 across the country, while some of them are quite locally 

 developed. When we proceed to correlate the subdivisions 

 of the rocks of the south-west of England with those in the 

 midland and north-eastern counties, many difficulties present 

 themselves. The variations in thickness, in lithology, and in 

 the assemblages of organic remains, are striking. When, 

 however, we look to the physical history of the period, and 

 the method of deposition of the sediments, such changes 

 appear but natural. It would be unreasonable to suppose 

 that an argillaceous, a sandy, or a calcareous deposit should 

 characterize any particular epoch of time, although any one 

 of these deposits might be laid down over a considerable area. 

 Such deposits were, no doubt, all being formed at the same 

 time. The great variations in thickness and other modifica- 

 tions exhibited by the Liassic and Oolitic series suggest that 



^ Tableau des Terrains qui composent I'Ecorce du globe, p. 221 ; see also 

 Buckland, Ann. Phil. xvii. (n.s. i.) 455 ; and Dr. A. Oppel, Die Juraformation 

 Englands, Frankreichs und des Siidwestlichen Deutschlands, 1856-58. 



