part 8] THEOUGII THE A:NDES or PEEU and BOLIVIA. 259 



to alternate repeatedly with fossilif erous limestones of Cenomanian 

 age, and interbedded lavas are frequently met with in similar 

 deposits, both in Southern Peru and in the district here described. 

 Moreover, since these rocks are themselves penetrated hj the 

 Tertiary diorites and granodiorites, it appears probable that they 

 constitute the first or volcanic phase of a great cycle of igneous 

 activity which commenced in Upper Cretaceous times, though 

 foreshadowed earlier, and was continued throughout the greater 

 part of the Tertiary Era to culminate in the series of plutonic 

 intrusions which foi-m the batholithic core of the Andes. In the 

 district covered by the section now described evidence of recent 

 volcanic activity is w^anting, though farther south volcanoes exist 

 which are not yet extinct. 



The western or Pacific slopes of the Cordillera are almost 

 entirely formed by porphyritic agglomerates and tuffs of the 

 volcanic facies, although a transition to the third or ' calcareous ' 

 facies is marked by the incoming at more than one locality of 

 limestones and associated quartzites. Beds of this nature, for 

 exam^ile, are seen dipping 75° south-westwards in the deep gorge 

 above San Mateo, known as the Puente del Infernillo, and, again, 

 forming an almost vertical junction with the agglomerate, at 

 Tambo de Viso, where the latter rock is pierced b}'' a broad dyke 

 of quartz-porphyry, ^o^vhere, however, do they appear to contain 

 fossils, nor does the calcareous facies become dominant until the 

 summit of the range is passed. 



At many places on the line of section other volcanic rocks, of 

 an intrusive character, are also met with, which are distinct from, 

 and obviously later than, those of 'contemporaneous' origin. These 

 occur chiefly in the form of andesite- and dacite-necks, laid bare by 

 erosion ; but in no case could the form of any cone or superficial 

 lava-stream be detected. 



In keeping with the previously-expressed view that in this 

 latitude the Andes have suffered their greatest compression, is 

 the fact that plutonic rocks of the granodiorite core are here 

 encountered almost at the summit of the mountain-range, more 

 than 15,000 feet above sea-level. Although these are not actually 

 exposed along the line of section, they crop out extensively a few 

 miles farther north in the Morococha Yallej'", where their relation- 

 ship to the overlying sediments and volcanic rocks is clearly 

 displayed. A brief description of the structure of this district 

 will serve to illustrate the part j^laj'^ed by these rocks in the 

 construction of the Cordillera. 



The valley of Morococha, now the site of important mining- 

 operations, was formerly buried beneath a great mountain-glacier, 

 the shrunken remnant of which occupies a small cirque at the 

 head of the valley not far below the limit of perpetual snow\ In 

 addition, several small tributary glaciers still cling to the steep 



