XI! 





cness of the consolidated masses. They may pass 



on the one hand, by admixture of ordinary sediment, into 



sandstones and shales; on the oth the coarse 



tumultuous agglomerate of purely volcanic origin, so 



commonly found filling up former vents of eruption. 



Igneous rocks may be conveniently classed as Plutonic, 



cp-seated, when they have solidified deep within the 



crust, and as Volcanic, when they have been erupted or 



the surface. Fundamentally, however, 



, ".'.,' V.V 



.-. - ^ i 



they are all parts of one great series. In the more super- 

 ficial or volcanic group they may either be intrusive or 

 intcrstratificd ; that is, they may either have been intruded 

 among the rocks with which they are associated, or they 

 may have been poured out at the surface in sheets, which in 

 a great continuous series of deposits thus come to be inter- 

 bedded between the strata below and those above them. 

 It is of course evident that as these crystalline masses 

 have all risen from molten reservoirs within the earth, they 

 were all originally intrusive in the earlier or deeper pan 



