50 MR. J. A. DOUGLAS 0>~ GEOLOGICAL SECTIOXS [yol. lxXVl, 



We now begin to enter the densely-forested region of the 

 Montana, and any attempt to determine the relations between 

 individual rock-exposures becomes well-nigh impossible. 



The metamorphic schists, however, can be more or less con- 

 tinuously followed as far as Casahuiri, varying in character from 

 time to time according to their original arenaceous or argillaceous 

 nature. Here they once more pass gradually into phyllites, slates, 

 and relatively unaltered sediments. 



Midway between Ollachea and Urahuasi is exposed a large surface 

 of rock having the appearance of a fault-plane : this dips steeply 

 southwards, and breaks across the planes of schistosity, which at 

 this point are inclined in exactly the opposite direction. Below 

 Urahuasi Bridge further outcrops of plutonic rock are encountered 

 which have the composition of a mica-syenite or durbachite. 

 Locally traces of gneissic structure are visible, but the rock has 

 evidently not shared to the full extent the dynamic metamorphism 

 to which the older rocks of the district have been subjected. 



Between Llinquipata and Puerto Seddon are numerous roadside 

 exposures of more or less unaltered sedimentary rocks, chiefly in 

 the form of black friable shales containing auriferous cpiartz-veins. 

 They are, as a rule, highly contorted, but have a fairly-constant 

 strike east-south-eastwards. Although these were closely examined 

 in an attempt to determine their age, no trace of fossils was dis- 

 covered until we reached the tributary valley of the Chaquimayo 

 River, where the waterworn pebbles and boulders of the stream- 

 bed were found to contain graptolites in some abundance. 



The bed-rock is here almost entirely concealed from view beneath 

 a thick clothing of tropical vegetation, and, although by following 

 the valley to its source, occasional specimens were obtained in situ, 

 no zonal sequence of the beds could be established. I am much 

 indebted to the late Prof. Charles Lapworth for making a report 

 on the graptolites from this locality, and he had very kindly pro- 

 posed to publish this in the form of an appendix to the present 

 paper. The fauna is essentially similar to that obtained by 

 Dr. J. W. Evans from the Caupolican district of Bolivia, and was 

 shown by Prof. Lapworth to be of Llanvirn age. 



Faunal List. 



Loganograptus logani Hall. - Glos.-tograiitus acantli us Elles & 

 Goniograptus sp. nov. Wood. 



Didymograptus bifidits Hall. var. Cryptograptus tricornis Hall, var. 



Didymograptus stabilis Elles & Amplexograptus confertus Lapworth. 



Wood. Amplexograptus coslatns Lapworth. 

 Phyllograptus angustifolius Hall. 



Petrogeaphical Notes. 



A 121. Elaeolite - syenite porphyry. Rio San Gaban. 



A coarsely-porphyritic rock, of which the phenocrysts form nearly half 

 the bulk. These consist of greenish-grey elasolite, showing hexagonal and 



