Vol. 70.] THEOUGH THE A]S*DES OF PERU AKD BOLIVJA. 15 



From this point as far eastwards as Mururata Bridge, where they 

 are well exposed along the river-banks, the rocks consist chiefly 

 of dark slates and shales with a general dip south-westwards ; 

 immediately beyond Huancani, however, they are seen to be 

 intensely contorted, and they also show signs of folding at the 

 second and third bridges on the trail to Coroico. 



In the neighbourhood of the hacienda of Mururata, the 

 shales undergo a remarkable change of character, becoming very 

 soft and friable, and paler in colour — such as lighter tints of grey,. 

 purple, and reddish hue, with frequently a silky sheen approach- 

 ing that of a phyllite. These beds also appear to agree closely 

 with the description given by Dr. Evans of the rocks met with 

 in the district of Caupolican, where they were found to contain 

 fragments of Upper Cambrian and Ordovician trilobites, identi- 

 fied by Mr. Philip Lake yPeltura, Symphysurus, Trinucleus y 

 Ogygia, etc.). 



Dr. Evans considers the peculiar nature of the paler shales to. 

 be due ,to lateritization. In the absence of direct palaeontological 

 evidence, I have coloured all the sedimentary rocks of the eastern 

 slopes as of Silurian age ; but, from comparisons with more northern 

 districts, there is every reason to suppose that both Cambrian and 

 Ordovician rocks are also represented. 



(b) The Oranitic Core. 



Owing to the inaccessibility of the giant peaks of the Eastern 

 Cordillera, the nature of the rocks of which they are built up 

 has always been a matter of dispute among South American 

 geologists. 



The presence of a granitic core has, of course, invariably been 

 recognized, from the occurrence of that rock in the form of huge 

 boulders in the alluvial deposits of the La Paz Yalley. D'Orbigny 

 first figured Mount Illimani as a huge mass of granite ; Forbes, 

 however, denied that this was the case, and represented it as 

 composed entirely of Palaeozoic sediments. Later research has 

 shown that neither author was strictly correct. 



Sir Martin Conwav has brought back gneiss and granite from 

 near the summit of the mountain, while the slopes are known to be 

 formed by Devonian slates and quartzites. Prof. Hauthal has 

 also proved this to be the case for Huaina Potosi or Caca-Aca on 

 the north and Quimsa Cruz on the south, the central core in each 

 case being composed of a coarse granite. 



The high peaks no doubt owe their existence to the resistance 

 offered by this rock to erosive agencies, for its outcrop is not 

 continuous along the whole smnmit of the range. 



Crossing the pass of Huacuyo, we meet with nothing but 

 Palaeozoic sediments until Unduavi is reached, some 5000 feet down 

 on the eastern side. Here indications are seen of granite cropping 

 out in the immediate neighbourhood ; and this is again the case 

 near the base of the steep zigzag trail below Huancani, known as 

 the ' Tunca Queuta ' or ' Ten Turns.' 



