28 ECONOMIC GEOLOGY 



No scientist has ever worked more indefatigably in the prod- 

 ucts of magmatic segregation than J. H. L. Vogt. His 

 classification gave us: (1) The segregation of the native metals; 

 (2) the segregation of metallic oxides; and (3) the segregation 

 of metallic sulphides. In the segregation of the metals, the pre- 

 cious metals, gold, silver, and platinum are formed. 



(1) Silver, the least frequently occurring as an original mineral, 

 is in such form and quantity as to lead to the conclusion that the 

 metal followed the law of solidification of magmas from the 

 basic to the acidic. Gold is found in considerable quantities in 

 the basic rocks as diabases and diorites, and in such acidic rocks 

 as the granites and syenites; examples of the latter are found in 

 the pegmatites of Australia and Alaska. The platinum is a 

 product of segregation from such ultra-basic rock masses as the 

 peridotites and the modern form or appearance is as grains of 

 platinum associated with the metamorphic products. In serpen- 

 tine platinum is almost always associated with the segregation 

 of chromite, although all chromite does not contain platinum. 

 The most important segregation of metallic iron is manifested 

 at Disco Island, on the western coast of Greenland where large 

 masses having the appearance of meteoric iron have been 

 discovered. It was not until the ore body was subsequently 

 discovered in place that its true nature was known, viz., segrega- 

 tion from the basic intrusive basalt. 



Cobalt and nickel are more or less common and wherever one 

 is found the other is present. Perhaps in New Zealand is found 

 the best illustration of ore bodies of cobalt and nickel segregating 

 from a molten magma. This generally carries a certain per- 

 centage of iron, so it becomes an alloy of cobalt and iron. 



(2) The second class of products segregating from molten 

 magma is the metallic oxides. Types of chromite and magnetite 

 are among the more important and more common. In many 

 cases the titanium is so abundant in magnetite as to lead 

 to the conclusion that the titanium oxide plays the role of an 

 acid, and the iron that of the base. Illustrations are common 

 in the formation of such ore bodies in the Adirondack Mountains 

 in New York. 



Another mineral sometimes segregating as an original mineral 

 is cassiterite. In general, however, it is associated with 

 pegmatite, an acid product in the solidification of some magma. 

 In certain sections the character of the ore body is such as to 



