ORIGIN OF ORE DEPOSITS 43 



body or rock mass by the influence of high temperature or by 

 shearing stresses of sufficient inten-ity to generate considerable 

 heat. The heat necessary for metamorphism may arise from the 

 int i usion of a fluid magma or from the internal heat of the earth. 

 Dr. F. W. Clarke, in his " Data of Geochemistry" cites the follow- 

 ing changes as the most important: 



(1) Molecular Rearrangement. By this process a pyroxene is 

 converted into an amphibole. 



(2) By Hydration. The conversion of a peridotite or a pyrox- 

 enite into a serpentine or steatite would represent the change. 



(3) By Dehydration. Limonite, 2Fe 2 O3,Fe 2 (OH)6, is converted 

 into hematite, Fe 2 Os, and bauxite, A1 2 O 8 ,2H 2 O, into corundum, 

 A1 2 3 . 



(4) Oxidation and Reduction. Through oxidation ferrous com- 

 pounds become ferric. Through reduction hematite, Fe 2 O 3 , be- 

 comes magnetite, Fe 3 O4. 



(5) Changes other than hydration produced by percolating solu- 

 tions. The transference of some cement into a sandstone and 

 its conversion into a quartzite would be an example. 



(6) Metamorphism by the action of gases and vapors, the so-called 

 "mineralizing agents." The process generates new minerals 

 and introduces new solutions. 



(7) Metamorphism by Igneous Intrusion. By this process new 

 minerals are developed in the metamorphic aureole. 



The ores associated with metamorphic rocks may be divided: 

 (1) Into those which occur as layers or lenticles in the crystalline 

 schists, and (2) those which occur as metamorphosed metaso- 

 matic deposits of irregular shape. 



In the crystalline rocks, hematite and magnetite are often of 

 commercial significance. The hematite is produced by the dehy- 

 dration of limonite and the magnetite by the reduction of hema- 

 tite or by the decomposition of siderite through a loss of carbon 

 dioxide. 



As sulphides, chalcopyrite, galenite, sphalerite and pyrite are 

 common in Scandinavia. While not of so great importance in 

 themselves alone, they have been the source of the material for 

 the enrichment of the lodes, traversing the district. The magne- 

 tite ore bodies of the Adirondacks, New York, represent a meta- 

 morphosed deposit in the older schists and gneisses. (See Figs. 

 32 and 33.) 



Another illustration may be cited in the famous hematites and 



