part 3] 



GEOLOGICAL SECTIONS THEOUGH THE AlfDES. 



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northern slopes of the valley, and terminate in large piles of 

 morainic material. It is not intended, however, to describe here 



the magnificent example of glacial topo- 

 graphy afforded by this district; and it will 

 be sufficient to state that the floor of the 

 valley now discloses a series of rock-steps 

 occupied by a string of 'paternoster' lakes. 

 The evidence afforded by glacially-striated 

 rock-surfaces shows that the ice must have 

 formerly been at least 500 feet thick. 



We are here concerned chiefly with the 

 upper part of the valley, and a view taken 

 from the south, looking directly across the 

 lake known as Huacracocha, shows a large 

 mass of dacite forming a jagged ridge, 

 clothed in places with glaciei's, terminating 

 in the peak of Yanasinga (see PI. XYIII). 



At the head of the valley, however, above 

 the Anticona lakes, well-defined Cretaceous 

 limestones and quartzites are clearly visible, 

 forming the denuded edge of what must 

 once have been an extensive covering be- 

 neath Avhich the igneous rock consolidated. 

 The latter floors the greater part of the. 

 valley, and forms the rock-steps below the 

 Anticona and Julia-Victoria lakes. 



If we now descend the valley to the next 

 rock-step below Huacracocha, known as the 

 Alapampa ridge, we once more encounter 

 stratified rocks, limestones, and quartzites, 

 dipping westwards beneath the dacite, and 

 forming the floor upon which it rests. At 

 the margin of contact the limestones show 

 pronounced evidences of metamorphism, and 

 in places the igneous rock is bounded hj a 

 distinct selvage composed of compact green 

 serpentine, fibrous chrysotile, and lime- 

 garnets. 



The mode of occurrence of this great 

 mass of dacite is clearly different from that 

 of the volcanic rocks found elsewhere in the 

 district. It has, in fact, none of the fea- 

 tures characteristic of a true lava, and, as 

 _ it is essentially hypabyssal in origin, it 

 '^ might more accurately be termed a da ci te- 

 ll porphyrite. It is a compact, even-grained, 

 • dark-grey rock, and, although it contains 

 small phenocrysts of felspar and ferromag- 

 nesian minerals, these never attain large 

 dimensions. It has obviouslv consolidated beneath a considerable 



