﻿I860.] 



FORBES — BOLIVIA AND PERU. 



39 



As seen in the accompanying Map, this formation extends from 

 the Lake Titicaca in Peru, southwards, nearly, if not quite, through 

 the republic of Bolivia, and possibly runs right into the Argentine 

 provinces to the back or eastern side of the volcanic range of the 

 Desert of Atacama, and everywhere presents the same characteristic 

 features. The cupriferous sandstones, for example, which are so 

 characteristic of this formation, show themselves all along this ex- 

 tent. Beginning their appearance in the north (in the district of 

 Puno), they are seen at the Pacheta (Section 2), then at Pisaca, Coro- 

 coro, Chacarilla, El Turco, Santa Barbara, San Bartolo, and even fur- 

 ther south ; and Mr. Villamil informs me that they are also visible 

 in the district of Andalgalla, in the Argentine republic, — a distance 

 of fully 500 miles from north to south. The breadth of country over 

 which they are found is probably from 50 to 80 miles across ; and the 

 Sections Nos. 1 and 2 show the general character of their transverse 

 section, exhibiting a series of longitudinal ridges, elevated to no great 

 height above the general level of the plateau, seldom higher than from 

 1500 to 2500 feet above it, and formed by a series of anticlinals 

 having a general strike of from N.10°W. to N.W. with varying 

 dip. 



A few of the more important observations of the strike and dip of 

 the strata met with in Sections Nos. 1 and 2 are here appended : — 



Hill east of Santiago de Machaca. Soft red sandstones, Strike. Dip. 



with red and white marls N.10°W. 15° E. 



Hill at San Andres de Machaca. Eed and white marls N.20° W. 16° N.E. 



Hill east of Nasacara. Red clay, with pebbles and 



compact red shales 20° N.E. 



Hill further east of Nasacara. Red and purple clays. . . 10° N.E. 



Pacheta de Antarini. Soft red sandstones and red marls N.W. 45° N.E. 



Laguna del Toro. Brown-red sandstones, with gyp- 

 seous beds N.W. 50° N.E. 



Little east of Laguna del Toro. Red-brown sandstones, 



with cupriferous beds N.W. 70° N.E. 



Pacheta. Red-brown sandstones, with cupriferous beds NN.W. 40° W. 



Pacheta (further east). Red and yellowish sandstones N.E. 30° S.E. 



Pacheta (still further east). Red and yellowish sand- 

 stones 20° S.E. 



Ditto. ditto 10° SE. 



Ditto. ditto 30° S.E. 



Ditto. ditto N.E. 20°NN.E. 



West of Tambillos. Light reddish-brown sandstones, 

 with finely contorted lamellar structure, and the 



stratification marked by rows of peculiar black spots NN.W. 25° N.E. 



ElTambillo. Red sandstones N.W. 45° N.E. 



Between El Tambillo and Coniri. Red conglomerate, 

 large pebbles of quartzite, grauwacke, and granite, 



occasionally a fragment of clay-slate N.W. 40° N.E. 



La G-uarclia (Section No. 1). Red sandstone and grey 



pebbly conglomerate, with colourless quartz pebbles NN.W. 45° S.W. 



East of Pisacoma. Thin fine red grey and white sand- 

 stones and pebbly conglomerates N. 10° W. 20° S.E. 



In the accompanying Map (PI. I.) it will be seen that at Penas, 

 near the south end of the Lake of Titicaca, a small patch is coloured 



