CHAPTER IV. 



IGNEOUS EOCKS, METAMORPHISM, SHRINKAGE AND DIS- 

 TURBANCE OF THE EARTH'S CRUST. 



I HAVE already explained that all rocks are divided into 

 two great classes, those of aqueous and those of igneous 

 origin; and I have shown how aqueous rocks may generally 

 be known by their stratification and by the circumstance 

 that a great many of them contain relics of marine and 

 freshwater life, in the shape of fossil shells, fish-bones, 

 and other kinds of organic remains. The materials 

 also of which these beds are composed generally show 

 signs of having been in water, being rounded by the 

 action of the waves of the sea, or by the running waters 

 of rivers. 



The other kinds of rocks, termed igneous, occasion- 

 allv are associated in different localities with the form- 

 ations named in the foregoing table. For instance, 

 there are no volcanic rocks in Wales associated with 

 the Carboniferous and Old Eed Sandstone strata, while 

 there are in Scotland, and true contemporaneous vol- 

 canic rocks are intercalated with the Lower Silurian 

 rocks of Wales and Cumberland, while there are none 

 associated with the equivalent strata in Scotland. 

 Some of these contemporaneous igneous rocks consist of 

 beds of volcanic ashes, others of old lavas, others of 

 masses of matter which were intruded among the strata 

 from below. Rocks that have been melted are known 



