ORES PRODUCED BY IGNEOUS PROCESSES. 1051 



Vog't, although so strong an advocate for igneous action, does not 

 include among ore deposits formed by eruptive after-action in connection 

 with pegmatite veins an)- gold, silver, copper, lead, zinc, or iron deposits. 

 There is therefore agreement between Vogt, Lindgren, and myself upon the 

 fundamental point that the great class of ore deposits which have been 

 known as contact metamorphic deposits were deposited by underground 

 solutions. There is also agreement that the igneous rocks in various 

 instances constituted the partial or chief source from which the metals are 

 derived. There is further agreement that the waters which deposited the 

 materials were hot, and derived their heat largely from the igneous rocks. 

 It follows from this that we agree that the ores were mainly put into 

 their present places during the time in which the igneous rocks were 

 able to produce a contact effect through hot solutions. There is also 

 agreement that the water depositing the ores was derived from two sources, 

 the ordinary circulating underground waters and water derived from the 

 crystallization of the magmas themselves. So far as the waters were 

 derived from the latter source, it is further agreed that in many cases they 

 were rich in metalliferous material, and therefore unusually effective. 

 Probably in some cases solutions from the igneous rock were more 

 important than those of the meteoric origin. 



In a genetic classification of ores upon the basis of agency there is no 

 place for the contact deposits. It does not follow that no ore deposits are 

 produced in connection with contact metamorphism. Contact metamorphic 

 ores must be produced by direct magmatic processes and therefore be 

 igneous ore deposits, by gaseous solutions, by aqueous solutions, or by 

 some combination of the three. In other words, the terms contact meta- 

 morphism and eruptive after-action are too vague to give any precise meaning 

 with reference to the deposition of ores. That this is so is shown by the 

 very different usages of the words "contact metamorphism" and "eruptive 

 after-action" by different authors. It is perfectly clear that different 

 deposits, or even the same deposit, produced in connection with contact 

 metamorphism or eruptive after-action may belong partly to igneous ores, 

 gaseous ores, and aqueous ores. Shortly after intrusion there may be 

 magmatic segregation along the border of the igneous rocks; later, in the 

 process of cooling, additions may be made to the ore deposits by gaseous 



