part 3] THRO L' OH THE AXJ)ES OF PERU AND BOLIVIA. 275 



proved at more than one locality. The more proiioimced meta- 

 morphism of the miea-scliists, as compai'ed with that of the 

 phyllites, maV; I think, he explained on the assumption that the 

 former represent rocks which had already undergone a certain 

 amount of change as a result of the granitic intrusion before being 

 subjected, together with the surrounding unaltered shales, to the 

 dynamic crushing Avhich later affected the whole series, including 

 the granite itself. 



Beyond Utcuyacu the outcrop of the granite cuts across the 

 strike of strongly-folded limestones, similar in character to those 

 above Palca. In the rock-cut on the trail between San Lorenzo 

 and the hill known as Pan de Azucar (sugar-loaf), these are seen to 

 contain casts of a brachiopod, tilled Avitli white calcite. These 

 casts are of large size, in many cases measuring 3 inches or more 

 in dejDth, and the concave-convex form of their cross-section is 

 strongly suggestive of a JProdiicfus of the ' gigontens'' type. 

 Although such evidence can hardly be claimed to be satisfactory, 

 it nevertheless niay be regarded as furnishing some support to 

 Prof. Steinmann's view that the limestones of Palca are of Carbo- 

 niferous age. 



As is the case in that localit}^, they are folloAved here bj 

 phyllites, the junction of the two rocks, owing to the intense 

 nature of the folding, being nearly vertical. These ph3dlites, 

 though obscured in places by the thick growth of forest vegetation, 

 can be traced as far as San Pamon, where, without any indication 

 of increased metamorphism, they overlie a coarsely-crystalline red 

 granite which was proved to crop out continuously to a point some 

 distance beyond the eastern limit of our section. 



This granite is a typically alkaline rock, and differs markedly in 

 composition from the granodiorites that are found in the heart of 

 the Cordillera. It is very uniform in character over a large tract 

 of country, and although, in this district, at least, there appears to 

 be a complete lack of positive evidence as to its geological age, the 

 presence of practically unaltered sediments in close proximity to so 

 enormous a mass of plutonic rock can hardly be accounted for, 

 except on the assumption that the origin of the latter was 

 antecedent to their deposition. 



It must, therefore, date from at least Lower Palaeozoic times, 

 and, as was suggested in the case of the plutonic rocks of the 

 Inambari district, it has evidently been an important factor in 

 checking the advance of the folded chains into the area of the 

 Brazilian platform. It may therefore be regarded as belonging to 

 an extra-Andean igneous province, characterized by rocks of 

 ' Atlantic ' facies. 



Petrograpliical Note. 



[Numerals in parentheses indicate microscopic slides and hand-specimens 

 preserved in the University Museum, Oxford.] 



(A. 152.) Granite. — Perene. — A coarse-grained, holocrystalline pink 

 rock of granitic texture, consisting almost entirely of quartz and felspar, to 

 which the coloured minerals are quite subordinate (sp. gr. = 2'.58). 



