GOLD AND SILVER DEPOSITS — LINDGREN. 155 



ver veins surround these intrusions, .and contribute from numerous 

 centers in the interior cordilleran region to the total production. 



Contact-metamorphic deposits, formed where limestone beds have 

 adjoined the igneous contacts and absorbed the emanations from the 

 intrusive magma, add their smaller share to the precious metal pro- 

 duction, but are usually richer in the base metals. 



Lastly we have a remarkable type of veins, which occur in lava 

 flows near volcanic vents, and which were formed near the surface 

 by hot springs charged with emanations from the molten rocks. 

 These deposits are often wonderfully rich, both in gold and silver. 

 They are the "bonanza" deposits proper; the Comstock, Tonopah, 

 Goldfield, and Cripple Creek are among the more celebrated locali- 

 ties of such veins; few of them are found north of the Canadian 

 boundary and none of them along the main Canadian or American 

 coast, but they are best represented in Nevada, Arizona, Utah, and 

 Colorado. In the United States they yield not less than $30,000,000 

 a year. 



Going farther south we enter the great mining region of the Mexi- 

 can plateau. For nearly 400 years an unceasing stream of silver has 

 been poured out of the mines of Mexico, and at the present time the 

 country produces annually about 2,000,000 kilograms, or 64,000,000 

 ounces of that metal. 1 Igneous rocks, both flows and intrusions, 

 abound in Mexico, and practically all of the deposits are of latest 

 Cretaceous or of Tertiary age, thus on the whole more recent than 

 any of those of Canada and the United States. 



The most celebrated silver mines are of the type formed in or near 

 volcanic flows near the surface. We need cite only Pachuca, Guana- 

 juato, and Zacatecas; but there are hundreds of other similar dis- 

 tricts. Of late the annual gold production has risen sharply to 

 $20,000,000 or $25,000,000; part of this comes from silver or base 

 bullion, but the greater part is derived from veins in volcanic rocks 

 similar to those just described and situated at El Oro, in the State 

 of Mexico. It should not be overlooked, however, that there are 

 also in the Cretaceous limestone countless though small intrusive 

 masses of diorite or porphyries around which auriferous or argentif- 

 erous veins or contact-metamorphic deposits have formed, and which 

 contribute their share to the production. 



THE ANTILLES. 



Evidences of a feeble mineralization are found in Cuba, Haiti, 

 Porto Eico, and Jamaica and more or less placer gold was obtained, 

 particularly during the sixteenth century from the first three islands 



1 The total production of silver in Mexico is estimated as 122,500 metric tons, a quan- 

 tity far greater than that yielded by any other country in the world. (See Beyschlag, 

 Krusch and Vogt (Die Lagerstatten, vol. ii, p. 69 (Stuttgart, 1912)). 



