THE JURASSIC PERIOD. 235 



thickness of from 150 to 200 feet. The Purbeck beds consist 

 of arenaceous, argillaceous, and calcareous strata, which can 

 be shown by their fossils to consist of a most remarkable alter- 

 nation of fresh-water, brackish-water, and purely marine sedi- 

 ments, together with old land-surfaces, or vegetable soils, which 

 contain the upright stems of trees, and are locally known as 

 " Dirt-beds. " 



One of the most important of the Jurassic deposits of the 

 continent of Europe, which is believed to be on the horizon 

 of the Coral-rag or of the lower part of the Upper Oolites, is 

 the " Solenhofeu Slate " of Bavaria, an exceedingly fine-grained 

 limestone, which is largely used in lithography, and is cele- 

 brated for the number and beauty of its organic remains, and 

 especially for those of Vertebrate animals. 



The subjoined sketch-section (fig. 159) exhibits in a dia- 

 grammatic form the general succession of the Jurassic rocks of 

 Britain. 



Regarded as a whole, the Jurassic formation is essentially 

 marine ; and though remains of drifted plants, and of insects 

 and other air-breathing animals, are not uncommon, the fossils 

 of the formation are in the main marine. In the Purbeck 

 series of Britain, anticipatory of the great .river-deppsit of the 

 Wealden, there are fresh-water, brackish-water, and even terres- 

 trial strata, indicating that the floor of the oolitic ocean was 

 undergoing upheaval, and that the marine conditions which 

 had formerly prevailed were nearly at an end. In places 

 also, as in Yorkshire and Sutherlandshire, are found actual 

 beds of coal : but the great bulk of the formation is an indu- 

 bitable sea-deposit; and its limestones, oolitic as they com- 

 monly are, nevertheless are composed largely of the commin- 

 uted skeletons of marine animals. Owing to the enormous 

 number and variety of the organic remains which have been 

 yielded by the richly fossiliferous strata of the Oolitic series, 

 it will not be possible here to do more than to give an outline- 

 sketch of the principal forms of life which characterize the 

 Jurassic period as a whole. It is to be remembered, however, 

 that every minor group of the Jurassic formation has its own 

 peculiar fossils, and that by the labors of such eminent ob- 

 servers as Quenstedt, Oppel, D'Orbigny, Wright, De la Beche, 

 Tate, and others, the entire series of Jurassic sediments admits 

 of a more complete and more elaborate subdivision into zones 

 characterized by special life-forms than has as yet been found 

 practicable in the case of any other rock-series. 



