part 1] THROUUIl THE ANDES OF PEKU AND BOLIVIA. 49 



only other rock of like character that I have come across in the 

 whole district is a white quartzite from the Carboniferous Series 

 in the neighbourhood of Puno, and I know of no record of beds of 

 later age than this occurring on the eastern slopes of the Cordillera 

 in this latitude. 



The sides of the river- valley almost as far as Ollachea are formed 

 of a coarse elreolite-syenite porphyry of very distinctive character. 

 It is a handsome rock, composed of large, grey, porphyritic crystals 

 of anorthoclase and ela^olite, set in a black or dark-grey micro- 

 crystalline ground-mass. Locally it has been subjected to intense 

 dynamic metamorphism, and has been transformed into a banded 

 gneiss. The conspicuous phenocrysts normally show no definite 

 arrangement ; but, where pressure has affected the rock, they 

 assume a roughly-parallel direction, individual crystals being 

 crushed and sheared in the process, first into elongate lenticles, 

 and finally into long drawn-out lines. 



The change, as seen macroscopically, is almost identical with 

 that described by Prof. H. J. Seymour 1 in the progressive 

 dynamo-metamorphism of a porphyritic andesite from County 

 Wicklow, the result in each case being a banded rock in which 

 original ground-mass and phenocrysts have given rise respectively 

 to alternating dark and light bands. 



Owing to the inaccessible nature of the gorge, the tectonic 

 relations of this rock are not clear ; but it appears to be intrusive 

 into the slates and metamorphic schists which succeed it lower 

 down the valley. 



A short distance above Ollachea hot springs issue on the left 

 of the valley, and deposits of siliceous sinter are being formed in 

 small though well-defined terraces. Near this point other and 

 distinct intrusions of plutonic rock are met with, in the form of 

 augite- and elasolite-syenites. The former is almost identical in 

 a hand-specimen with certain essexites, but microscopically it is 

 seen to be more closely related to a syenite of the laurvikite type. 

 The elasolite-rock, in which the coloured minerals are onfy sub- 

 ordinate, closely resembles foyaite. 



Both of these intrusions appear to be of later date than the 

 elseolite-sj^enite porphyry mentioned above, for, microscopically at 

 least, they show no signs of having been subjected to dynamic 

 action. This whole suite of igneous rocks is of typical ' alkaline ' 

 facies, and may be regarded as forming part of the great Brazilian 

 province, which is thus shown to extend, as indeed might have 

 been expected on a priori grounds, to the very flanks of the folded 

 chains. 



Sedimentary rocks, comprising dark slates and phyllites. are met 

 with for the first time in the immediate neighbourhood of Ollachea, 

 and when these are followed down the valley they are seen to pass 

 into biotite-schists containing abundant rose-pink crystals of 

 andalusite. 



1 Sci. Proc. Royal Dublin Soc. n. s. vol. xi, pt. 5 (1902) p. 568 

 Q. J. G. S. No. 301. e 



