﻿I860.] 



FORBES BOLIVIA AND PERU. 



7 



November 21, 1860. 



Eobert Jones Garden, Esq., 63 Montagu Square ; and Robert 

 Home, Lieut. R.E., Royal Staff-College, Sandhurst, were elected 

 Fellows. 



The following communications were read : — 



1. On the Geology of Bolivia and Southern Peru. 

 By David Forbes, Esq., F.R.S., E.G.S., &c. 



[Plates I., II., III.] 



Contents. 



Introduction. 



1. Tertiary and Diluvial Formations 



of the Coast. 



2. Saline Formations. 



3. Diluvial Formations of the Inte- 



rior. 



4. Volcanic Rocks. 



5. Dioritic Rocks. 



6. Upper Oolitic Series with inter 



stratified Porphyritic Rocks. 



7. Permian or Triassic Formation. 



8. Carboniferous Formation. 



9. Devonian Formation. 

 10. Silurian Formation. 



Introduction. — In laying before the Society a statement of the ob- 

 servations made during an examination of Peru and Bolivia, in the 

 years 1857, 1858, 1859, and 1860. I may observe that the present 

 memoir is to be considered as the first part of a report of the results 

 obtained during my travels in South America during these years ; 

 and, consequently, it is believed that the conclusions here arrived at 

 will have more weight when considered in conjunction with the ob- 

 servations on the geology and mineralogy of the neighbouring re- 

 publics of Chile and the Argentine provinces, which subsequently I 

 shall have the honour to lay before the Society, — more particularly 

 as several of the geological formations not well developed or examined 

 into in the district forming the subject of this memoir exhibit them- 

 selves much more characteristically further south. 



Many points will be found not so elucidated or examined into as 

 could be desired, and might appear to have been neglected ; this, 

 however, has not arisen from oversight, but is due to the great dif- 

 ficulties and frequently severe privations encountered in exploring a 

 country in many parts entirely uninhabited or in next to a savage 

 condition, and further by my having been limited as to time and 

 pecuniary resources, and hampered by other occupations and by the 

 political state of the country. 



In the construction of the accompanying Map and Sections (Plates 

 I. & II.), Nos. 1 & 2 of which give a good idea of the structure and 

 formation of the different mountain-ranges of the Andes, the hori- 

 zontal distances are laid down from the best local information which 

 could be procured, and from data furnished by the Bolivian Govern- 

 ment-survey lately completed. For the vertical altitudes in addi- 

 tion to those determined by myself barometrically, and occasionally 

 by boiling-point and trigonometrical observations, I have employed 

 some of the heights noted on Mr. Pentland's map, — and further the 



