602 



SCIENTIFIC RECREATIONS. 



solidified be considered, the differences will be fully accounted for. The 

 greenstones and syenites are prominent amongst the plutonic series. 



These plutonic and volcanic rocks are separated into basic and acidic, 

 as already remarked, but the line cannot be drawn very distinctly. Granite is 

 the chief plutonic (acidic) rock, and we frequently find it forced upwards into 

 other strata, its essentially eruptive character being thus decided. That 

 granite must have taken an immense, time to solidify and crystallize is evident, 

 for no new granites are ever found. We find granite in all the old mountain 



chains such as the Grampians 

 in Scotland, and the Wicklow 

 mountains. Our chief Euro- 

 pean (active) volcanoes are, so 

 to speak, modern, as may be 

 supposed when their consti- 

 tuents are known. It may be 

 said that granite was first de- 

 posited as sediment heated by 

 subterranean fire, and forced 

 up by thermal action of water 

 to the mountains, where it is 

 uncovered by a slow process of 

 denudation and surface wash- 

 ings of the earth. 



Now without at present 

 goingany farther into the causes 

 of volcanoes we can see at a 

 glance that the eruption of the 

 igneous rocks must have created 

 a marked and essential differ- 

 ence in the physical geography 

 of the globe. It is to these erup- 

 tions that the dislocation and 

 disturbance of the stratified for- 

 mations are due. The igneous 

 rocks present ridges in the 

 mountains ; sometimes they 

 are rounded at the summits, 

 while the aqueous and meta- 

 morphic rocks are disposed in layers. 



These two classes in their varieties form the land and the crust of the 

 earth, which is ever being acted upon by air and water. The ice, again, 

 polishes and scratches the valleys in which it moves. The loosened boulders 

 that tumble fromthe mountains are carried down by the ice, and deposited in 

 the glacier moraine, whence flows a stream. By degrees the stone is ground 

 up, and carried away in the water to form sediment in a " delta" at the 



. Crater of Popocatapetl. 



