66 ECONOMIC GEOLOGY 



rock has undergone very great distortion. (3) Massive ore: 

 Where few, if any, traces of either the original structure or the 

 original constituents of the Algonkian rocks can be observed. 



Cordilleran Region. This vast area stretches through British 

 Columbia on the north southward to Mexico and includes prac- 

 tically all States traversed by the Rocky Mountains. Its best 

 development is in Colorado. Each of these western States is 

 subdivided into fields and districts, and each is worthy of a de- 

 tailed description but only a few of the most important fields are 

 considered. 



Cripple Creek. This district is situated 10 or 12 miles from 

 Pikes Peak, Colorado, but in the foot hills of the same mountain 



FIG. 47. Small vein of andesitic breccia, Independence mine, Cripple 

 Creek, Colorado. An, Andesitic breccia; P, pyrite; F, fluorite; Q, quartz; 

 V, valencianite. (After W. Lindgren, U. S. Geological Survey.) 



mass. The field is the most important as a gold producer in the 

 Cordilleran belt. Its importance is testified to by the fact that 

 it has already produced more than $200,000,000 in gold. It is 

 essentially a gold field for the ore contains from 1 to 10 oz. 

 of silver per ton. 



The district consists of a series of highly metamorphosed mica 

 schists bearing sillimanite, of pre-Cambrian age; the Pikes Peak 

 granite, characterized by its microcline; the Cripple Creek granite 

 also bearing microcline; and the Spring Creek granite which carries 

 the commonest of the feldspars, orthoclase. In the metamor- 

 phics there also appear some differentiation products of an olivine- 

 syenite magma. The volcanics of the area consist largely 



