Ill 



KOUS ROCKS 



75 



not injected as an intrusive sheet Evidence of thi 

 nature abounds among the volcanic rocks of the Lover 

 Old Red Sandstone of Scotland (Fig. 69). In 



... t 



. - . -...' . ... ..;.> 



.1 . 



examples, the discharge of the lava was succeeded by a 

 period of volcanic quiescence, during which ordinary 

 sediment was strewn over the bottom of the water. 

 Sometimes the lava-beds in an old geological formation 

 are succeeded by beds of volcanic tuff, or these two kinds 

 of rock are intercalated with each other in such a way as 

 to show that streams of lava and showers of dust and 

 stones must have been erupted too rapidly to permit of 

 the accumulation of any prominent beds of ordinary 

 sediment between them. The way in which lavas and 

 tuffs sometimes alternate in a series of ancient strata is 

 well illustrated in Fig. 64, which represents the charac- 



