

1 NKOUS ROCKS 



It is obviously important, therefore, to be able to 

 termine satisfactorily whether igneous rocks hire been 



contemporaneously poured out during the deposition of 



the formation in im h they occur, or have been injected 



: tat formation after its accumulation, The existence 



of unmistakable beds of tuff </, /. Fig. 64) would settle 



the question. These rocks consist of ejected volcanic 

 detritus, and must have been laid down at the surface ; 

 they could not be injected as in the case of crystalline 

 rocks. 1 Whenever, therefore, we encounter interstratifi 

 cations of volcanic tuff in a group of sedimentary forma- 

 tions, we are justified in regarding them as evidence of 

 contemporaneous volcanic activity. The observer should 



1 Exception*! CAMS occur where the tuff that filU a volcanic vent 

 hat found iu way into crack, in the wall* of the funnel 



