PRECIOUS MKTM.S <>:>> 



tant field in the Black Hills. It has been a steady producer of gold 

 formany years. The Homestake ore bodies occur in the Algonkian 

 slates which are for the most part of sedimentary origin. One 



variety has however been recognized as a ineiani(.rplioM-d i^m-ou- 

 roek and catalogued as an amphibolite because amphibole is the 

 mo-t prominent constituent. The amphibolites occur as dikes or 

 irregular ma->es in the other Algonkian rocks. The associated 



FIG. 45. Banded siliceous ore in No. 2 shaft Union mine, Black Hills 

 district, South Dakota, showing preservation of sedimentary bedding in 

 the ore, the banding being continuous with the inclosing stratified rocks. 

 (After J. D. Irving, U. S. Geological Survey.) 



metamorphosed sediments are quartzites, quartz-schists, mica- 

 schists, phyllites and graphitic, garnetiferous and chloritic slates. 

 More recent eruptives cut through all these rocks as well as 

 the ore bodies themselves. The later eruptives are of two types. 

 (1) A rhyolite porphyry, which is by far the most common rock. 

 It not only cuts through the Algonkian terranes but spreads out 

 in sheets or sills in the nearly horizontal strata of the overlying 

 Cambrian series. (2) The second eruptive is a trachytoid phono. 



