254 JURASSIC. 



their subdivisions were not formed contemporaneously, but 

 that a clayey condition, such as that which produced the 

 Upper Lias, may have prevailed longer in one tract than in 

 others, and the same may be said of the Midford Sands. The 

 great thickness of Fullers' Earth in some places would like- 

 wise indicate a period of time which, in tracts where it has 

 thinned away, was occupied by the deposition of rocks of a 

 different lithological nature. Thus the conditions of sedi- 

 mentary deposit may have changed at different times in 

 different areas, and these changes would be marked, as a rule, 

 by the modifications in the assemblage of organic remains 

 which were suited to the then prevailing physical conditions. 

 The clayey beds, it should be observed, are more persistent 

 than the sandy or calcareous strata. Sir A. C. Ramsay con- 

 siders that the Liassic and Oolitic formations were sediments 

 laid down in warm seas surrounding an archipelago of which 

 Dartmoor, Wales, and Cumberland formed some of the 

 islands. The finer sediments were, no doubt, brought down 

 by rivers, whose former presence is indicated by estuarine 

 beds ; the calcareous sands and freestones have been formed 

 in part from comminuted shell and coral : while evidences of 

 coral-reefs occur at some horizons.^ 



On the whole, the Jurassic System stands out in marked 

 contrast from the strata of the preceding New Red System, 

 whose rocks furnish but little attraction to the collector of 

 fossils, although the Rhaetic beds may be said to usher in 

 the earliest traces of Liassic life. The flora of the Jurassic 

 period includes Conifers and Cycads ; and the fauna, many 

 Corals, Echinoderms, Crustacea, Insects, Mollusca, Fishes, and 

 Reptiles among which are the earliest Crocodilia found in 

 this country.^ Mammals likewise occur. 



Among Mollusca, the Ammonites make their first appearance 

 in this country, and they form a most important and striking set of 

 fossils. It has been remarked that "The beds of closely-packed 

 Ammonites, of every stage of growth, which occur in certain of the 

 Jurassic rocks, would appear to be due to the effect of occasional 

 rapid earthy deposits, which took place during that seasonal 

 period, when the Molluscs, lying torpid and contracted within their 

 shells, were at once entombed in that condition." ^ Belemnites are 

 especially abundant in the Jurassic clays. ^ 



^ See H. C. Sorby, Addresses to Geol. Soc. 1879, 1880. 



2 A. Smith Woodward, G. Mag. 1SS5, p. 496. 



^ Morris and Lycett, Gt. Ool. Mollusca, p. 3. 



■* See Monographs of the Palseontographical Society, by Owen, Phillips, Wright, 

 Davidson, and others ; also A. D. D'Orbigny, Paleontologie Francaise : Terrain 

 Jurassique ; and L. Agassiz, Poissons Fossiles, 1833-43, Etudes critiques sur las 

 Mollusques fossiles, 1840. 



