164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



monzonitic porphyries not at all connected with the " effusives " or 

 flow rocks. 



It is thus clear that practically all of the Peruvian deposits are of 

 the intermediate type, formed far below the surface. It is doubtful 

 whether there are in Peru any deposits of the type of the Tonopah, 

 Comstock, or Pachuca veins. 



The great Cerro de Pasco deposits, for instance, occur in or close 

 to a stock of " dacite " or " biotite andesite," which has metamor- 

 phosed the surrounding Cretaceous sediments. The proper name 

 would seem to be biotite-diorite porphyry. In their upper levels the 

 veins carried probably secondary silver ores of wonderful richness, 

 while in depth they have been found to contain low-grade copper 

 ores, which now form the basis of a great industrial enterprise. 



Besides the smaller bodies of intrusive porphyries, there are also 

 numerous large intrusive masses or " batholiths " of granodioritic 

 rocks. Some of these form the central parts of the great ranges, and 

 they may continue for a long distance with a width sometimes reach- 

 ing 50 miles. Around these also there has been more or less minerali- 

 zation, but of a more feeble character than attended the intrusion of 

 the porphyries. The time of intrusion is taken to be early Tertiary. 



In the gold-bearing region of southeastern Peru (northeast and 

 north of Lake Titicaca) we find different conditions. Here the 

 folded sedimentary rocks are of early Paleozoic age and more or less 

 intruded by porphyries and granodiorites. This is in the regions of 

 Carabaya and Sandia, and the Inambari Basin on the montana 

 slope. A very widespread, though not intense, mineralization has 

 taken place; the primary gold deposits are apparently poor but the 

 placers are widely distributed and numerous; partly successful at- 

 tempts have been made to mine them. This belt is, in fact, the 

 northern continuation of the great tin-silver-gold belt of the eastern 

 range of Bolivia. 



BOLIVIA. 



Bolivia produces little gold at the present time, but its placers on 

 the montana side have at times yielded heavily. They lie on the 

 eastern slope of the great range, east of Lake Titicaca, which counts 

 among its peaks Sorata and Illimani, each over 21,000 feet in eleva- 

 tion. Celebrated among these were the placers of Tipuani on the 

 east slopes of Sorata, which have yielded great amounts of gold since 

 the time of the conquerors. There are many other localities south 

 of this. Other placers have been worked recently on the San Juan 

 River near the Argentine boundary. At the present time only two 

 gold veins are worked, both in the eastern range and said to be of the 



