150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



the present time. At the same time masses of molten rock have been 

 forced up from great depths into the rocks nearer the surface, and 

 cooled there to granites and diorite porphyries without ever reaching 

 the surface, though through gradual wearing away of the covering 

 rocks many such masses are now exposed at the surface. 



Almost all primary gold and silver deposits have been formed 

 during or shortly after epochs of volcanic or intrusive activity. 

 Secondary deposits are derived by the disintegration and concentra- 

 tion by water of such primary deposits. They are called placers or 

 alluvial deposits and are usually cheaply and easily worked. 



On the American Continent the primary- gold and silver deposits 

 date from two widely separated ages. The first period is geologi- 

 cally very ancient and belongs to the pre-Cambrian or early Paleo- 

 zoic; its deposits are thinly scattered over the entire continental area, 

 but are at many places covered by later rocks. The second period is 

 much more recent, and belongs to the late Mesozoic and the Tertiary. 

 Its numberless deposits were formed during the great igneous activity 

 which accompanied the building of the Cordilleras and are thus 

 confined to the western or cordilleran part of the continents in which 

 area the deposits of the older period are rare because capped by later 

 sediments or flows. 



Placers may be formed from deposits of either period. 



1. DEPOSITS OF THE EARLY PEKIOD. 



Naturally, the deposits of the early period are best observed in the 

 great eastern expanse of the continents where the early rocks are 

 often splendidly exposed. Gold is the principal metal and is always 

 accompanied by quartz gangue. The deposits bear evidence of hav- 

 ing been formed at considerable depth and high temperatures. While 

 the majority of these occurrences are poor, yet great richness may be 

 found in small areas, and the purity and coarseness of the gold is 

 favorable to the formation of placers, especially in temperate or 

 warm climates. Wherever continental ice sheets have covered a re- 

 gion, as in Canada, they have almost invariably ground up and 

 scattered the placers. 



NORTH AMEBIC A. 



In North America deposits of this kind are formed in the southern 

 Appalachian States, in South Dakota, in Quebec, in Nova Scotia, and 

 in Ontario. In the latter Province the recently discovered Porcupine 

 district presents a case of extraordinary richness, the annual pro- 

 duction being now over $4,000,000. The celebrated Homestake mine 

 in South Dakota is working on a pre-Cambrian replacement deposit 

 in form of thick lenses of altered schist with free gold. While con- 

 taining only about $4 per ton, the ores yield annually over $5,000,000. 



