154 ANNUAL REPOBT SMITHSONIAN INSTITUTION, 1917. 



time contribute to the gold production of South America about 

 $8,500,000, or not far from the amount extracted from the same class 

 of deposits in North America. 



2. DEPOSITS OF THE LATER PERIODS. 

 GENERAL FEATURES. 



From Cape Horn to Alaska the gold and silver deposits of the 

 cordilleran belt are formed under similar geological conditions, and 

 are of the same general geological age. It hag already been empha- 

 sized that they are products of the igneous activity which has ac- 

 companied the rise of this gigantic mountain chain. 



They were formed within several epochs, but all of them lie be- 

 tween the earliest Cretaceous and the present; that is, they are late 

 Mesozoic, Cenozoic, or Quaternary in age. They were formed, on 

 the whole, nearer to the surface than the old deposits of the pre- 

 Cambrian, or at least under conditions of more moderate tempera- 

 ture. Many of them, indeed, were formed very close to the present 

 surface. Following intrusions or lava flows, hot waters loaded with 

 gases and metals of igneous origin rose toward the surface, and, in 

 cooler regions of the crust, deposited their load of metals. In part 

 the gold and silver occur in minute quantities associated with cop- 

 per and lead minerals, and are recovered from the base bullion. 

 Much silver is obtained in this manner, but most of the gold is de- 

 rived from gold quartz deposits, properly speaking, or from placers 

 caused by the wearing down by erosion of these deposits. 



NORTH AMERICA. 



It is difficult indeed to give in a few paragraphs even an approxi- 

 mate idea of the gold and silver deposits of the North American 

 cordillera. The annual yield of the region is enormous, attaining 

 now $130,000,000 in gold and nearly $100,000,000 in silver. 



A great gold-producing belt lies along the Pacific and reaches 

 from California to Alaska, with local interruptions. These are the 

 oldest deposits of early Cretaceous age and they have yielded vast 

 placer or secondary deposits. The annual production, including 

 the placers, is not less than $40,000,000. Geologically they are con- 

 nected with the intrusion of dioritic rocks, an intrusion extending 

 like a gigantic dike along the Pacific coast mountains. 



Throughout the interior part of the cordilleran region are num- 

 berless smaller intrusions of granitic or dioritic rocks, or of the 

 porphyries of these rocks, most of them of earliest Tertiary age, 

 some a little earlier, others a little later, Aureoles of gold and sil- 



