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Gold-silver, silver-lead, copper, and antimony deposits are described. 

 Two epochs of mineralization are recognized, the first following the 

 intrusion of a group of granitoid rocks in late Cretaceous or early Ter- 

 tiary time, the second following the extrusion of a group of later Ter- 

 tiary andesites and rhyolites. Most of the ores are believed to date 

 from the former epoch. The gold-silver and silver-lead ores are vein 

 deposits. Most of the copper ores are replacements of sedimentary 

 rocks in the vicinity of monzonitic intrusives. 



A number of good examples of magmatic differentiation are described. 



C. W. T. 



"A Geologic Reconnaissance of the Cuzco Valley, Peru." By 



Herbert E. Gregory. Am. Jour. Sci., XLI (1916), pp. 



1-100, pis. 2, figs. 44. Contributions from the Peruvian 



Expedition of 19 12 under the Auspices of Yale University and 



the National Geographic Society. 



The bottom of Cuzco Valley ranges from 10,000 to 11,000 feet 



above the sea. The higher peaks of its bordering ranges reach heights 



of from 14,000 to nearly 16,000 feet. Between 13,000 and 13,500 feet 



above the sea, there is a well-defined plateau surface, called by Gregory 



the "Inca peneplain." It is characterized by much gentler slopes than 



those of the deep canyons which have been incised into it by the present 



streams. It is underlain for the most part by complexly folded and 



faulted sedimentary rocks. Quaternary glaciation molded all of this 



region above 13,500 feet, and occasional moraines descend to 12,500 



feet. 



The Cuzco Basin, occupying the upper end of the valley, is about 

 ten miles long by three miles wide. It is believed to owe its origin to 

 faulting. It is floored with lacustrine sediments, probably of pre-glacial 

 age, which are overlapped by large alluvial fans, now being dissected. 

 At the head of the valley is a plateau surface sloping from 11,500 feet 

 up to 13,000 feet above sea-level, underlain by Cretaceous limestones 

 carrying marine fossils — the only marine sediments known in this 

 region. The great bulk of the mountain masses surrounding the valley 

 is made up of clastic sedimentary rocks: brown sandstones, chocolate 

 shales, and conglomerates. These are divided by Gregory into three 

 formations, one of which is composed chiefly of pyroclastic material. 

 They are tentatively assigned to the Permian and Jura-Trias. There 



