THE GEOLOGICAL RECORD 203 



The Triassic Rocks continue the story begun by the Permian. 

 They are sandstones, conglomerates, and marls, with the remark- 

 able addition of beds of rock-salt and alabaster or gypsum The 

 coarse deposits indicate rapid denudation, the well-rounded sand 

 grains (Fig. 18) and unfossiliferous rocks point to desert conditions 

 and deposit under terrestrial conditions, and the rock-salt and 

 gypsum to the evaporation of such inland salt lakes as are often 

 found in desert regions. As would be expected, evidence of life is 

 rare, being confined to a few rare plants, entomostraca, scorpions, 

 and reptiles. The last are represented not only by bones and 

 teeth, but by tracks (Fig. 20), some of them of dinosaurs remark- 

 ably birdlike in their organisation and footprints. But the most 

 remarkable fossils hitherto obtained are portions of what is 

 probably the oldest known mammal. In this connexion it is 

 important to notice that in South Africa and elsewhere remains 

 of two extinct orders of reptiles presenting many affinities with 

 the mammals have been found. The vegetation is characterised 

 by an increased proportion of cycads. In the Alps the strata 

 occupying the position of the Trias are of marine character with 

 abundant and well-preserved shells, crinoids, corals, and foram- 

 inifera, mostly of types which become more common in the 

 succeeding Formations, but including not a few survivors of 

 Palaeozoic types. The sandstones of the Trias give excellent 

 building stones, and they yield one of our most important supplies 

 of water. The salt beds and brine springs are our chief source 

 of salt, and the gypsum beds are worked for plaster and impure 

 alabaster. The rocks occur for the most part in great plains 

 like those of Chester, York, and the Midlands, which are some- 

 times deeply trenched by rivers. The land upon them is gener- 

 ally fertile, and occupied by arable, forest, and gardens. 



The rocks of the Jurassic System are mostly alternations of 

 oolitic limestone and clay (Figs. 13 and 29), the former much 

 quarried for freestone, the latter excavated for brick-making. 

 They were deposited in a warm sea of Mediterranean type, at first 

 deepening on the whole, but afterwards shallowing and eventually 

 becoming dry. Fossils are abundant and well preserved. The 

 most conspicuous types are ammonites and belemnites belonging to 

 the cephalopods ; abundant reptiles like ichthyosaurs and plesio- 



