1062 A TREATISE ON METAMORPHISM. 



deposited to make the material an ore. The gangue of such veins, the 

 barren veins, and the cement of the tuff are connected physically and are 

 similar mineralogically. Thus it is certain that they are the result of like 

 processes, and few can doubt that the general cement and vein filling were 

 deposited through aqueous solution. 



The conglomerates and amygdaloids of Lake Superior furnish con- 

 clusive evidence that the deposition of the copper was a mere incident 

 in the general process of cementation. Throughout the Lake Superior 

 region the Keweenawau amygdaloids, sandstones, and conglomerates 

 have been cemented by the minerals which developed in the belt of 

 cementation. Throughout this region, coincident with this process, there was 

 deposited sparsely disseminated copper. Scarcely a Keweenawau district 

 has been studied in which such disseminated copper, associated with the 

 cementation minerals, may not be seen, and in many cases it is sufficiently 

 abundant to warrant exploration with the hope of finding' ore deposits. In 

 only a single district, however, that of Keweenaw Point, has copper been 

 discovered in sufficient quantity in veins, amygdaloids, and conglomerates 

 to constitute ore deposits. In every respect these copper deposits are like 

 the remaining great volume of the Keweenawan greenstones, sandstones, 

 and conglomerates, being cemented in substantially the same manner and 

 with the same minerals. At various places on Keweenaw Point from 1 to 

 4 per cent of metallic copper was deposited simultaneously with the other 

 cementing minerals. There can be no escape from the conclusion that the 

 general cementation of the Keweenawau rocks and the deposition of the 

 copper were performed by the same agent and at the same time. I do not 

 know that any one has ever held that this general cementation is the result 

 of other than aqueous solutions. If this general belief be true it is conclu- 

 sively shown that the copper was also deposited by aqueous solutions. 

 Other districts similar to those mentioned could be cited. 



Passing to veins of lead, -zinc, gold, and silver it is found that all but 

 a comparatively small number of such veins exhibit the cementation min- 

 erals which have been mentioned. The ores are . so intimately associated 

 with these gangue minerals that they must have been simultaneously depos- 

 ited. In many cases it can be shown that the filling of the veins was con- 

 temporaneous with extensive alterations of the walls within which developed 

 like minerals, and with the general cementation of the belt of cementation. 



