part 3] THEOUGH THE AliDES OF PEETJ AWD BOLIA^IA. 279 



towards the north by a Hmestone facies that has clearly been laid 

 down under true marine conditions. Evidence of a somewhat 

 similar nature may also be cited from the Jurassic rocks. In 

 Central Peru no younger beds than the Lower Lias were met with ; 

 farther south at Lagunillas on the Arequipa-Puno railway deposits 

 of Inferior Oolite age are found, while at the Moito de Arica in 

 the extreme north of Chile the series is continued into the Oxf ordian. 

 Althougli this distribution might be partly accounted for as a 

 result of pre -Cretaceous erosion, in my opinion it is largel}^ due to 

 differential uplift which in this case commenced earlier in the north 

 than in the south. 



Turning now to the actual sequence of deposits, we find that 

 Pala30zoic sediments older than the Devonian are everywhere con- 

 lined to the eastern flanks of the Andes. Fossils are of rare occur- 

 rence in these rocks, and no attempt has up to the present been 

 made to establish their detailed subdivision. A definite belt of 

 Ordovician graptolite-bearing shales (of Llanvirn age) appears, 

 however, to be more or less continuous from Bolivia to the 

 Inambari district in Peru. Farther north these beds are possibly 

 represented by the altered phyllites described in the foregoing 

 jDaper. 



Fossiliferous deposits of Lower and Middle Devonian age are 

 of wide extent in the region of Lake Titicaca and the Bolivian 

 Altaplanicie. Their extension north of Cuzco is unknown, but 

 the}^ appear to be wanting in the Chanchomayo district of Central 

 Peru. 



Owing to the absence of LTpj^er Devonian and Lower Carboni- 

 ferous rocks over the whole of the country, a period of orogenic 

 uplift appears to have set in at this time, the uplift being probably 

 accompanied by the intrusion of the granite of the Eastern 

 Cordillera of Bolivia. Both the syenitic rocks of the Inambari 

 area and the granite of Utcuyacu described above, though possibly 

 of earlier date, may also have had their origin at this time. All 

 these rocks show evidence of subsequent dynamic metamorphism. 

 This feature, however, cannot be explained solely by their age, for 

 younger rocks have shared with them compression and folding, 

 possibly of even greater magnitude than that of the earlier move- 

 ment, with but little alteration. 



Their position with reference to the general trend-lines of the 

 Cordillera must also have been a determining factor : for it is a 

 significant fact that, almost without exception, the metamorphic 

 rocks are confined to the maroins of the folded chains awav from 

 the axis of major uplift. Here, situated close to the resistant 

 horsts which form the 'jaws of the vice,' they have been exposed, 

 without experiencing the relief obtained by uplift, to the full force 

 of the compression. Lower Carboniferous rocks have been recorded 

 only at Yiscachani, between Tirapata and Ollachea, where I dis- 

 covered a fauna indicative of the extreme tojD of the Avonian 

 sequence. They possibly also occur in the Palca district of Central 

 Peru, but definite proof of their existence has not yet been 

 established. 



