CHAPTER XV 

 UNSTRATIFIED OR MASSIVE ROCKS 



The unstratified or massive rocks have risen in a molten state 

 from below toward the surface, though by no means always reach- 

 ing it, and have forced their way through or between the stratified 

 rocks. One of the most important points to determine with 

 regard to a massive rock is its relation to the strata in which it 

 occurs ; for the earth's chronology is given by the stratified rocks. 

 Considered only with reference to itself, an igneous mass gives no 

 trustworthy evidence as to the time when it was formed. The 

 term eruptive is frequently employed in the same sense as unstrati- 

 fied, because of the belief that most igneous masses have been 

 connected with volcanoes ; but as such a belief may not be well 

 founded, it is better to use a non-committal term. 



We shall first take up the volcanic rocks, because modern 

 volcanoes give us the key by which we may readily interpret 

 them. 



I. Ancient Volcanoes and their Rocks 



Volcanic Necks. — Volcanoes, like all other mountains, are sub- 

 ject to the destructive effects of the atmosphere, rivers, and the 

 sea. In an active volcano the upbuilding by lava flows and frag- 

 mental ejections more than compensates for the loss by weather- 

 ing, and the cone continues to grow in height and diameter. 

 When the volcano has become extinct, the destructive agencies 

 work unopposed. We find extinct volcanoes in all stages of 

 degradation, from those which look as though their activity 

 might be renewed at any moment, to those which require' the 

 careful examination of a skilled geologist to recognize them for 

 what they are. 



274 



