GOLD AND SILVER DEPOSITS LINDGREN. 159 



Steinmann's profiles * from the Pacific to Rio Marailon show 180 

 kilometers of Upper and Lower Cretaceous beds with interbedded 

 volcanics, strongly folded and in part overturned toward the east, 

 There are in these Cretaceous rocks numerous early Tertiary in- 

 trusions of granodiorite and porphyries ("Andesitische Tiefenges- 

 teine"), but few of them are more than 10 kilometers in width. In 

 the valley of the Maranon, old (" pre-Devonian") schists and gran- 

 ites appear for the first time and probably form the continuation 

 of the pre-Cambrian of Colombia and Ecuador. The porphyritic 

 intrusions are extremely numerous, and Steinmann refers to them 

 as "laccoliths," though usually they have a vertical attitude, con- 

 formable to the surrounding sediments. Farther south the gran- 

 odioritic batholiths become even more abundant, one exposed in the 

 Rimac River being 50 kilometers in width. They always meta- 

 morphose the surrounding Cretaceous limestone. 



BOLIVIA AND SOUTHERN PERU. 



A section across this region, recently described by J. A. Douglas, 2 is 

 330 kilometers long, but does not include the whole of the montana 

 slope. Here the Andes are divided into the western cordillera, the 

 Bolivian high plateau or the "Altiplanicie " and the eastern cordillera 

 or the Cordillera Real. The latter includes the highest summits, Illi- 

 mani and Sorata, but contains no volcanoes or large masses of vol- 

 canic rocks, It is largely built of older Paleozic sediments (Cam- 

 brian, Ordovician, Silurian, and Devonian), mostly slates and sand- 

 stones, and these are intruded by masses of granite, diorite, and por- 

 phyries. The Upper Devonian and the lower Carboniferous are both 

 absent. 



In the Altiplanicie we find the same folded Paleozoics, with trans- 

 greding Cretaceous in part terrigenous sediments, such as those of 

 Coro-Coro. The Cretaceous is covered by post-Miocene andesites. 



The western Cordillera along this section is essentially a volcanic 

 range with numerous dormant or extinct volcanoes, and vast accumu- 

 lations of lavas, including rhyolite, trachyte, and andesite. 



Underlying these rocks and beautifully exposed along the Chilean 

 coast as far north as Arica are Jurassic and Cretaceous strata inter- 

 bedded with contemporary lavas and intruded by early Tertiary 

 granular rock. The latter range from quartz monzonite to quartz 

 diorites, and are accompanied by pegmatite dikes, many of which 

 carry tourmaline. These intrusives are best exposed in the canyons. 



1 Gebirgsbildung unci Massengesteine in der Kordillere Sudamerikas (Geologische 

 Rundschau, vol. i, Fas. 1—3, 1910). Ueber gebundene Erzgange in der Kordillere 

 Sudamerikas (International Mining Congress, Diisseldorf, 1910). 



2 Section across the Andes in Peru and Bolivia (Quarterly Journal of Science, vol. lxx, 

 Pt. I, pp. 1 to 53 (1914)). 



