104 THE GEOLOGY OP MINNESOTA. 



[Tuff. 



space not plainly embraced within the feldspars. There is no remaining evidence 

 of the nature of the other original minerals. 



A better section, made by Marchand, shows distinctly a tuffaceous nature. 

 There is a portion of the microlitic magma, composed of spicules of plagioclase 

 surrounded by devitrified glass, now greenish, and a portion made up of a minutely 

 fragmental rock, which, between crossed nicols, is nearly dark, but plainly contains 

 numerous angular qinirf.^'^. In the former are large translucent areas which are 

 contrasted with the dark surrounding rock, and in these areas is seen much of a 

 finely granular gray mineral, having high refraction and also high double refraction, 

 which probably is epnlotf. This mineral is associated with much quartz in these 

 areas, and also is disseminated throughout the rest of the rock in more minute particles. 



Two sections. 



Aye. Cabotian. 



Remark. This marks the first discovery of tuffaceous rock in the series that 

 has been called Cupriferous, in the lake Superior region. It appears by the following 

 descriptions that volcanic tuff is not an uncommon substance in the region of 

 Duluth, and at points further east. N. H. w. 



The following criteria for the recognition of ancient volcanic rocks have been 

 given by Dr. George H. Williams (Journal of Geology, ii, page 10): 



Criteria for the ri'mijnition of ancient volcanic rocks. 



It is a self-evident proposition that the identification of certain rocks as volcanic products is in no way 

 dependent upon their present association with a recognizable crater or volcanic mountain. By volcanic rocks 

 we understand igneous or pyroclastic material which has solidified or been deposited at or very near the earth's 

 surface. It is of little moment whether or not it was ever piled into conical mountains. That the rocks them- 

 selves bear witness to their origin and conditions of formation is sufficient. The successive effects of erosion on 

 the easily removed volcanic mountains has often been so graphically described* that no further reference to the 

 subject is here necessary. If the Eocene or Triassic volcanoes have so disappeared as to leave 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 bo proved, only such regions are here considered 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 in many cases where this at lirst appears to be tin- 

 fact, some decisive clue may be discovered. 



In establishing the volcanic nature of rocks occurring in ancient and more or less crystalline terranes, 

 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 deciding 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 volcanic: 



1. They may be found in the field to occur in distinct sheets, flows or necks. 



2. They will have produced little or no contact action in the adjoining rocks. 



* See DE LA BECHE : Gmloijical observer, pp. 526-537, 1851. M. NEUMAYER : Erdgetchictite, vol. i, pp. 202-204, 1887. W. M. 

 DAVIS : " The Lost Volcanoes of Connecticut." Popular Science Monthly, December, 1891. 



