Ch. XXI.] ORIGIX OF THE OOLITE AXD LIAS. 429 



this era * The absence as yet from the Lias and Oolite of all signs 

 of dicotyledonous angiosperms is worthy of notice. The leaves of 

 such plants are frequent in tertiary strata, and occur in the Cretaceous, 

 though less plentifully (see above, p. 335). The angiosperms seem, 

 therefore, to have been at the least comparatively rare in these older 

 secondary periods, when more space was occupied by the Cycads and 

 Conifers. 



Origin of the Oolite and Lias. — If we now endeavor to restore, in 

 imagination, the ancient condition of the European area at the period 

 of the Oolite and Lias, we must conceive a sea in which the growth 

 of coral-reefs and shelly limestones, after proceeding without interrup- 

 tion for ages, was liable to be stopped suddenly by the deposition of 

 clayey sediment. Then, again, the argillaceous matter, devoid of corals, 

 was deposited for ages, and attained a thickness of hundreds of feet, 

 until another period arrived when the same space was again occupied 

 by calcareous sand, or solid rocks of shell and coral, to be again suc- 

 ceeded by the recurrence of another period of argillaceous deposition. 

 Mr. Conybeare has remarked of the entire group of Oolite and Lias, 

 that it consists of repeated alternations of clay, sandstone, and limestone, 

 following each other in the same order. Thus the clays of the lias are 

 followed by the sands of the inferior oolite, and those again by shelly 

 and coralline limestone (Bath oolite, &c.) ; so, in the middle oolite, 

 the Oxford clay is followed by calcareous grit and coral-rag ; lastly, in 

 the upper oolite, the Kimmeridge clay is followed by the Portland 

 sand and limestone.f The clay beds, however, as Sir H. De la Beche 

 remarks, can be followed over larger areas than the sands or sand- 

 stones.]; It should also be remembered that while the oolitic system 

 becomes arenaceous and resembles a coal-field in Yorkshire, it assumes 

 in the Alps an almost purely calcareous form, the sands and clays 

 being omitted ; and even in the intervening tracts it is more compli- 

 cated and variable than appears in ordinary descriptions. Neverthe- 

 less, some of the clays and intervening limestones do retain, in reality, 

 a pretty uniform character for distances of from 400 to 600 miles from 

 east to west and north to south. 



According to M. Thirria, the entire oolitic group in the Depart- 

 ment of the Haute Saone, in France, may be equal in thickness to 

 that of England ; but the importance of the argillaceous divisions is 

 in the inverse ratio to that which they exhibit in England, where they 

 are about equal to twice the thickness of the limestones, whereas, in 

 the part of France alluded to, they reach only about a third of that 

 thickness. § In the Jura the clays are still thinner and in the Alps 

 they thin out and almost vanish. 



* Tableau des Veg. Foss., 1849, p. 105. 



f Con. and Phil., p. 166. 



% Geol. Researches, p. 337. 



§ Bnrat's D'Aubuisson, torn. iii. p. 456. 



