22 Igneous Rocks. 



by heat. The crystalline structure identical with or 

 similar to some modern lavas, the occasional columnar 

 structure, the amorphous earthy look, also common in 

 certain lavas, the slaggy, ribboned, and vesicular struc- 

 tures, the penetration of strata by dykes and veins, 

 and the alteration of the stratified rocks at the lines 

 of contact, all prove the point. 



Modern volcanic ashes are simply fragments, small 

 and large, of lava ground often to powder in the crater 

 by the rise and fall of the steam-driven rocky material. 

 This is finally ejected by the expansive force of steam, 

 and with the liberated vapour, volcanic dust, lapilli and 

 blocks of stone, are sometimes shot thousands of feet 

 into the air mingled with watery vapour, which con- 

 densing in the higher atmosphere, falls with the ashes 

 on the sides of the volcanic cone in heavy showers of 

 rain. By the study of modern volcanic ashes, it is, after 

 practice, not difficult to distinguish those of ancient 

 date, even though they have become consolidated into 

 hard stratified rocks. Their occasional tufaceous char- 

 acter, the broken crystals, the imbedded slaggy-looking 

 fragment of rocks and bombs, and sometimes the 

 occurrence of coarse volcanic conglomerates, every frag- 

 ment of which consists of broken lava, all help in the 

 decision. In fact, tracing back, from modern to 

 ancient volcanoes, step by step through the various 

 formations, the origin of ancient volcanic rocks is clear ; 

 and further, it leads to similar conclusions with respect 

 to the igneous origin of bosses of crystalline rocks, such 

 as some granites, syenites, and dioritic masses which, 

 having been melted and cooled deep in the earth, were 

 not ejected, and never saw the light till they were ex- 

 posed by denudation. 



