HE strata of the present system are eminently distinct 

 from those of the former, in the composition and colour 

 Limestone Ridges at Leuctra. o f the beds, and in the numerous and unique organisms 



unfolded to our view. We are presented indeed with much the same general 

 compounds argillaceous, arenaceous, and calcareous deposits clays, sandstone and 

 limestone rocks but they are very readily distinguishable from those of older date 

 by their texture and hues. The clays have commonly a deep blue tinge. The sandstones, 

 chiefly calcareous, through an admixture of carbonate of lime, have a cream- yellow tint, 

 derived from the oxide of iron, in* a different state of oxidation to that which the decided 

 red of the preceding formation indicates. The limestones are of various shades, but 

 remarkably characterised by a concretionary or shelly structure, which is due to the 

 collection of particles of lime by molecular attraction around shells, corals, or grains of 

 sand. In order to produce this diverse series of strata, we must suppose some great 

 change to have transpired in the condition of terrestrial nature, from that during which 

 the magnesian rocks, red sandstones, and saliferous marls were deposited. The upheaval 

 of these latter formations from the floor of the existing oceanic basins, creating new land 

 to be disintegrated, and submerging portions of dry ground, thus forming a coast-line of 

 fresh strata, to be abraded by the tides and currents of the sea this is the physical 

 revolution which is suggested. Though we can never know the precise alterations made 

 by the event, in the relations of land and sea, yet it renders quite intelligible to us the 

 building up of the oolitic rocks beneath the waters of the deep, from the sediments 

 introduced to them from the changed terrestrial superficies. But, especially, these 

 deposits most strikingly contrast with thpse of previous origin, by their organic remains, 

 those of saurian animals, which in great numbers and gigantic size inhabited the rivers 

 and estuaries, and also occupied their banks, during the period of their formation. From 

 this circumstance, that interval, embracing a long series of ages, has been styled " the 

 age of reptiles." 



