16 ME. J. A. DOUGLAS OS GEOLOGICAL SECTIONS [vol. lxxvi, 



stresses reached a maximum, relief appears to have heen afforded 

 by fracture, accompanied by the intrusion of a second type of 

 igneous magma, for at the points where the gneissic structure 

 is most completely developed (kilometres 145 & 149, west of 

 Uchumayo) the rock is seen to be broken through by large basic 

 intrusions in the form of a medium-grained green augite-diorite 

 (A 84). 



The constant occurrence of the latter rock in localities where the 

 granodiorite has assumed a gneissic structure, can, I think, only be 

 explained in this way, for the contact of the two rocks is distinct, 

 and there does not appear to be any admixture of the two types, 

 nor can the gneissic structure developed in the granodiorite be 

 explained as having been formed by a process of injection. 



The diorite consists chiefly of felspar, augite almost completely 

 converted into a fibrous green actinolite, and a deep-brown biotite. 

 Between kilometres 131 and 132 the granodiorite is again pene- 

 trated by a rock of somewhat similar character (A 77), paler in 

 colour and with a considerable amount of unaltered pyroxene 

 present. 



The diorite forms the sides of the railway-cuttings and the river 

 gorge between Uchumayo and Huaico (kils. 149—160). Like 

 the granodiorite, it is frequently traversed by small veins of 

 quartz and epidote, and near kilometre 153 it is associated with 

 rocks resembling phyllites, which have the appearance of highly- 

 metamorphosed sediments. 



Near the station of Tiabaya, between kilometres 160 and 164 

 and again at kilometre 167, it is itself penetrated by a medium- 

 grained grey granite or adamellite (A 85), representing the third 

 and last phase of plutonic intrusion. This rock is composed of 

 clear quartz, white felspar, hornblende, biotite, and small yellowish- 

 brown crystals of sphene. Epidote, so abundant in the other 

 members of the series, is typically absent. In general appearance 

 it is not unlike the granite described above as cropping out in the 

 desert near Huagri ; but, owing to the isolated position of the 

 latter, the correlation of the two is only a matter of conjecture. 



Near the zone of contact the granite contains numerous basic 

 inclusions of diorite, and it is obvious that these are fragments 

 plucked off and partly absorbed, in the nature of accidental xeno- 

 liths ('enclaves enallogenes ' of Prof. A. Lacroix). 



The diorite shows clear signs of contact-metamorphism, chief of 

 which is the formation of green hornblende in crystals of large 

 size ; it has also been considerably acidified by admixture with the • 

 granitic magma. 



Locally, near the margin of contact, it assumes a finely-banded 

 appearance, with the development of abundant minute fiakes of 

 biotite. This character, owing to its limited extent, is more 

 probably the result of fluxion of the diorite, rendered viscous by 

 the intrusive granite, than the effect of dynamic pressure ; the 

 microscopic features of the rock, in fact, are such that it appears 

 to have been completely melted and recrystallized. 



