46 ME. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [April I914, 



Prof. Hauthal describes the granite of the Eastern Cordillera 

 as occurring in the form of a laccolite ; but the evidence for such 

 -a mode of origin does not seem to be very strong, and its intrusion 

 in the form of a batholith appears to be more probable. With 

 regard to the age of the intrusion, there is a considerable dif- 

 ference of opinion. It was believed by Forbes to be of Middle 

 Silurian age ; but both Prof. Steinmann and Prof. Hauthal discuss 

 the possibility of its intrusion in Tertiary times. Apart from 

 the metamorphism of the Devonian and Silurian rocks with the 

 production, according to Prof. Bergt of Leipzig, of andalusit- 

 Jiornfels and knotenglimmerschief er, little evidence is 

 available. The rock differs essentially in mineralogical characters 

 from the post- Cretaceous granodiorites of the Western Cordillera, 

 and in my opinion there seems to be no valid reason for regarding 

 it as other than of Palaeozoic age. 



Since marine Lower Carboniferous and Upper Devonian rocks 

 appear to be absent in this district, it would appear that elevation 

 •of the land took place during those periods, and the movement was 

 possibly accompanied by the intrusion of plutonic rock. Such 

 .a view, however, is purely conjectural, and further evidence must 

 be sought before any exact determination of the age of the rock 

 can be attempted. 



(A 130 ) Granite ; Unduavi (Bolivia). 



Macroscopic characters: An even-grained white rock of 

 ;granitic texture, composed essentially of clear quartz, white 

 felspars, and black biotite. 



Microscopic characters : Quartz abundant, felspars 

 turbid, due to alteration into sericite ; orthoclase and acid 

 plagioclase with fine twin-lamellation (chiefly albite and oligo- 

 'dase). Several large crystals also occur in the section, show- 

 ing peculiar patchy extinction : these appear to be perthitic 

 intergrowths of orthoclase and albite. Biotite abundant, 

 ■dark reddish-brown and strongly pleochroic, much altered into 

 green chlorite and some epidote. It contains numerous in- 

 clusions of zircon with intensely pleochroic halos, and apatite. 



The rock also contains abundant fibrous aggregates of musco vite, 

 which appear to be due to the decomposition of cor die rite : some 

 of the latter mineral remains unaltered, and exhibits brown pleochroic 

 halos ; it is also associated with brown and green spinellids. 



V. General Summary and Coxclusioxs. 



The foregoing paper gives an account of the geological observa- 

 tions made during travels through the country comprising the 

 frontiers of Chile with Peru and Bolivia, a summary of the results 

 of which will serve to give a brief outline of the development of 

 this part of the Andes during jmst geological ages. 



In Cambrian, Ordovician, and Silurian times, the sea covered 

 that part of the country which is now occupied by the eastern 

 slopes of the Bolivian Cordillera. In which direction lay the land 



