F. L. Hess—Tactite. 377 



Aet. XXY. — Tactite, the product of Contact Metamor- 

 phismf by Feaxk L. Hess. 



Probably every geologist who has worked among con- 

 tact metamorphosed rocks has felt the need of a single 

 word by which to designate the alteration product that 

 has resulted when soluble rocks have been acted on by 

 gases, solutions and heat accompanying the intrusion of 

 granitoid masses. I have felt this need particularly in 

 a study of the contact metamorphic tungsten deposits of 

 the United States, in which I have been associated with 

 Mr. E. S. Larsen. 



In describing the altered rock it became very irksome 

 to continually refer to the "contact metamorphosed 

 rock" and to try to find some variation of the phrase 

 that would mean the same thing. The tungsten miners 

 in western Nevada and eastern California have shown 

 the need of such a term by christening the rock ' ' garnet- 

 ite," for in some localities the deposits are composed 

 largely of garnet, but some of the contact metamorphic 

 tungsten deposits contain little or no garnet, and the 

 word is for such rocks a misnomer. Thus, a contact 

 metamorphic tungsten deposit in the Grouse Creek 

 Mountains of northwestern Utah is mostly clinozoisite, 

 and garnet, if present, is a rarity. In other deposits 

 there may be a more or less great preponderance of mica, 

 epidote, diopside hornblende, or even pyrrhotite, accom- 

 panied by numerous minor minerals. In other types of 

 contact metamorphic deposits the prevailing mineral 

 may be, at least locally, quartz, calcite, specular hematite, 

 magnetite, chalcopyrite, pyrite or vesuvianite. With all 

 the variations, however, that the deposits may show from 

 place to place, even in parts of the same altered mass, the 

 characteristics are so plain to the geologist acquainted 

 with this type of rock that it is at once recognizable. 



The minerals mentioned are largely formed from ma- 

 terials introduced by the solutions that caused the meta- 

 morphism, and form a zone beyond which is another zone 

 that has a simpler mineralogy in which wollastonite or 

 tremolite, and recrystallized calcite are the principal 

 minerals. This zone is of little economic interest, but 

 the zone receiving the bulk of introduced materials is in 



* (Published by permission of the Director, XJ. S. Geological Survey.) 



