62 ECONOMIC GEOLOGY 



quartz and somewhat cellular from the oxidation of pyrite. The 

 veins were filled from ascending gold-bearing solutions. The 

 pyrite would serve as a reducing agent for the gold thus held in 

 solution. 



The principal mining ground is in placers which, according to 

 E. T. Hancock, may be divided into three classes. (1) The 

 gravel of the stream and bottom lands, deposited by fluviatile 

 action. (2) The gulch and hill-side deposits or accumulations 

 due to disintegration and motion induced by frost action and 

 gravity. (3) The upper decomposed layers of the country rocks 

 in place. 



The Virginia belt extends from North Carolina in a northeast- 

 erly direction to Montgomery County, Maryland, and is from 

 10 to 20 miles in width. The terranes consist largely of mica 

 schists and gneisses, often garnetiferous, talcose and chloritic. 

 The auriferous veins conform largely to dip and strike of the 

 schists. They are very irregular, lenticular and narrow, seldom 

 exceeding a few feet in width. The chief gangue is quartz but 

 the wall rocks are often impregnated with auriferous pyrite. 

 The Fisher lode in Louisa County is the most persistent. This 

 lode has been opened for a distance of more than 5 miles, 

 but the maximum depth to which the lode has been worked is 

 approximately 250 ft. 



Black Hills District. The Black Hills are situated in Lawrence 

 County in the western part of South Dakota. The auriferous 

 ores are in the northern Black Hills. The gold of South Dakota 

 was first discovered in the placers which occupy depressions in the 

 pre-Cambrian schists. The region in one of peculiar interest for 

 it represents some of the earliest known and worked placers of the 

 United States. After the placers became somewhat exhausted at 

 the surface, the workings were carried downward into the con- 

 glomerate that marks the base of the Cambrian series of rocks. 



Some geologists are of the opinion that the origin of the placer 

 gold is from the reefs formed by the Homestake ledge in the 

 Cambrian Sea. Other geologists consider that the gold was 

 chemically precipitated by the action of the sulphides of iron and 

 therefore not a true detrital deposit. A reason for this conclusion 

 lies in the fact that the matrix of the auriferous conglomerate is 

 pyrite rather than quartz; also that the gold occurs along frac- 

 ture planes stained by iron oxides. 



Homestake District: The Homestake belt is the most impor- 



