IGNEOUS ROCKS 125 



not very perfect, it is true, of many of the minerals which occur 

 in the crystalline rocks. It is therefore inferred that masses of 

 such rocks mark the site of either a once active volcano like Etna 

 or Vesuvius, or an unsuccessful attempt to establish such a volcano. 



Sometimes these crystalline rocks may be traced upwards, and 

 seen to pass into sheets running parallel to and enclosed between 

 beds of rock which look somewhat like sediment. The rock of the 

 sheets may be streaky in aspect, and may show hollows like the 

 bubbles or steam-holes seen in the lavas of existing volcanoes ; 

 and such rocks will show columnar or spheroidal shrinkage joints 

 also linking them with the lavas which they so closely resemble in 

 composition and texture. The associated beds, in the irregularity 

 of their bedding, the angularity of their fragments, and the fact 

 that the latter are either broken crystals or bits of crystalline 

 rock, may be compared more closely with the beds of ash and tuff 

 deposited from a modern volcano than with ordinary sediments. 



The character and association of the crystalline rocks suggests 

 that the whole of them are the product of volcanic action. This 

 is merely the escape of heated molten rock from the interior of 

 the earth, which either pours out in lava floods, breaks up into 

 tuff and ash thrown up by steam escaping from the lava, or 

 consolidates as a crystalline rock in the fissures through which 

 it is making its way from the interior of the earth to its surface. 



The interior of the earth is proved to be in an intensely heated 

 condition. Hot springs rising from a great depth, and the fact 

 that the temperature in wells, tunnels, or borings, steadily increases 

 from above downwards, provide some of the proof required. 

 Volcanoes testify to further activity, and the fact that volcanic 

 and crystalline rocks of many different ages form part of the 

 earth-crust proves that molten matter from the interior con- 

 tributes to form a considerable and important part of the earth- 

 crust. Volcanoes are found to be geographically associated with 

 lines of weakness in the earth-crust such as coast-lines, or with 

 mountain chains where the rocks have been lifted to a great 

 height above the average level. It is in such chains that the 

 folding of rocks reaches its maximum, showing that the energy 

 which has manifested itself in crushing the rock together and 

 folding it, has also squeezed out liquid matter from inside the earth. 



