192 THE WORL3>, 



15. VOLCANIC BOCKS. These are the products of fire, or sub- 

 terraneous heat, ejected from beneath the surface, through fis- 

 sures in the earth's crust, both in ancient and modern times ~ 

 The erupted materials of the ancient ^volcanoes being trap, basalt, 

 loadstone and tuff, and the products of recent sub-aerial volcanoes, 

 lava, scoriae,, pumice and ashes. 



The general proportionate thickness of each of these several 

 deposits has been estimated as under, but the statement must be 

 regarded as a men.' approximation. 



Tertiary System, ....,..., 2,000 feet. 



Cretaceous, 1,000 * 



Weald ...1,000 " 



Oolite and Lias, 2,500 



Saliferous, 2,000 



Carboniferous, 10,000 



Old Red Sandstone 10,000 



Silurian, 7,500 



Cambrian, 30,000 



Mica Schist, and Gneiss, not ascertained, but far exceeding that 

 of any of the superposed deposits. 



We have now given a connected view of the order of suc- 

 cession of the several strata, each characterised by its peculiar 

 animals and plants. All these are marine deposits except-one, 

 the fourth, called the Wealden. This is a fresh water formation, 

 and is the deposit of a mighty ancient river, or of several of them, 

 and its organic remains are such as might be expected to result 

 from the sediment of such a river, consisting of plants, shells, 

 fish, and reptiles, imbedded in the mud together. It is almost 

 the only evidence which remains of the ancient land, showing 

 that while the immense deposits were going on in the bed of 

 the ocean, here were bodies of fresh water, rolling over a vast 

 extent of land, bearing upon their waters the remain's of trees, 

 and huge reptiles. In the following chapters we shall consider 

 each of these formations more fully, and describe more particu- 

 larly some of the fossils found in them. It will be observed by 

 the careful reader, that most of the marine deposits of the several 

 epochs have the same mineralogical character: if we except the 

 coal, we will find the rest alternating with marls, clays, lime- 

 stones, and sandstones ; each being formed from the ruins of 



