THE DISTRIBUTION OF ANCIENT VOLCANIC ROCKS. 1 1 



only traces of their original forms, what may we expect of those 

 of Paleozoic or Archean times? 



On the other hand, the association in dissected volcanic 

 regions of the effusive rocks with correspondingly abyssal types 

 naturally suggests that volcanoes may have once surmounted 

 many areas of coarsely granular ancient igneous rocks. As this, 

 however, cannot be proved, only such regions are here con- 

 sidered as yield rocks of unmistakably surface origin. 



Again, ancient volcanic rocks may have been subjected to 

 metamorphosing processes severe enough to have destroyed 

 most of their original characters. In such cases, patient study 

 and a careful weighing of all evidence is necessary to decide 

 their origin, and even that may not avail. Igneous rocks may 

 be so altered as to be indistinguishable from metamorphosed 

 sediments, but it many cases where this at first appears to be the 

 fact, some decisive clue may be discovered. 



In establishing the volcanic nature of rocks occurring in 

 ancient and more or less crystalline terrains, attention must be 

 given to several different sets of characters. The field relations 

 must be carefully studied and the material collected on the spot 

 and afterward studied in the laboratory. The criteria for decid- 

 ing on their igneous and volcanic origin may be arranged as 

 follows : 



I. If the rocks are igneous^ whether abyssal or surface, they 

 will : 



1. Conform in chemical composition to certain well 

 established types ; 



2. Show an association of petrographical types which, 

 both chemically and mineralogically, follow the laws 

 of consanguinity. 



II. If they are volcatiic : 



1. They may be found in the field to occur in distinct 

 sheets, flows or necks ; 



2. They will have produced very little or no contact 

 action jn the adjoining rocks ; 



3. They may include irregular fragments of other rocks. 



