IGNEOUS ROCKS. 



571 



Rhone. The same effect can be traced where the latter river enters the 



Lake of Geneva. So the land is being perpetu- 

 ally carried away and deposited ; and where water 

 gains on land there is somewhere else always a 

 corresponding elevation to compensate it. Thus 

 places disappear, and the sea washes over the site, as 

 on the Kentish Coast, where Earl Godwin's land was 

 inundated, and new land is reclaimed or is ele- 

 vated from the sea to make up the balance. 



Fig. 644. Trap rock (Stafia). 



" There rolls the deep, where grew the tree ; 

 Oh, earth, what changes hast thou seen ! 

 There, where the long street roars, has been 

 The stillness of the central sea. " 



We have spoken of sedi- 

 mentary and organic rocks. There 

 is yet another kind called igneous, 

 or fiery rocks those upraised by 

 volcanic action. Of the igneous 

 rocks the crystalline have been 

 evidently in a fused condition. Granite is an example ; lava or basalt is 

 the usual term for volcanic rock, and the basaltic caves of Staffa and the 

 Giants' Causeway bear testimony to the igneous or volcanic origin of the sur- 

 roundings. The pillars and fantastic rocks of Ireland and Scotland which 

 are so remarkable, are simply lava, which was erupted in a molten state, now 

 cooled and contracted into blocks of curious regularity of form. 



Granite, already referred 

 to, is another igneous rock, 

 and must have been forced 

 upwards; for as an igneous 

 rock granite has cooled be- 

 neath the crust of the earth, 

 throwing out arms, while melt- 

 ing, into other formations, 

 and frequently being found in 

 mountains. There is another 

 kind of igneous rocks formed 

 by the continuous accumulation 

 of the ashes, etc., vomited 

 forth from volcanoes. Masses 

 of mountain are thus produced 



, - . A . Fis- 645. Eruption of granite. 



in the course of years, and the 



material thus formed is called tufa, or tuff, when consolidated ; and this 



(now solidified) is what caused the destruction of Pompeii and 



Hcrculaneum. 



So we have two classes of IGNEOUS ROCKS, the Crystalline and 



