276 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. Ixxvii, 



Microscopic characters. — Quartz and felspar make tip the bulk of the 

 rock, the coloiTred minerals, with the exception of a little magnetite, consisting" 

 solely of secondary haBmatite. 



The felspar consists of perthitic intergrowths of orthoclase and albite, the 

 latter showing very narrow twin-lamellae and a refractive index considerably 

 lower than that of quartz. Maximum extinction-angle (in sections cut perpen- 

 dicular to the lamellse) = 11°. 



Ferromagnesian minerals typically absent. 



Zircon in small brilliantly -polarizing prisms is the only other accessory 

 mineral represented, in addition to the magnetite. 



(A. 151.) Granite. — Utcuyacu. — A medium-grained white rock of 

 granitic texture, consisting chiefly of quartz, white felspar, and biotite 

 (sp. gr. = 2-68). 



Microscopic character s. — Quartz is very abundant. 



The felspars comprise both orthoclase and plagioclase, the former being 

 slightly in excess of the latter, although the relative proportion varies some- 

 what in different parts of the same slide. A small amount of microcline with 

 typical cross-hatching is also present. The bulk of the plagioclase lies 

 between oligoclase and andesine ; it shows low extinction-angles and a 

 refractive index very close to that of quartz. 



The ferromagnesian minerals comprise chestnut-brown biotite. showing 

 occasional bending of the lamellae, and a smaller amount of green hornblende. 



Accessory minerals comprise sphene and zircon. 



The above- described rock differs from the typical granodiorites of the 

 Cordillera in the following respects : — The greater abundance of quartz ; the 

 presence of microcline ; the much smaller proportion of ferromagnesian 

 minerals ; and the complete absence of iron-oxides. 



(A. 150.) Granitoid Gneiss. — Near Huacapistana. — A specimen 

 of the Utcuyacu granite in which typical gneissic structure has been 

 developed. 



Microscopic character s . — The large allotriomorphic crystals of quartz 

 seen in the above rock have been broken up into a granular mosaic. 



The plagioclase shows fracture and distortion of the twin-lamellfe. 



The hornblende seems to have entirely disappeared ; but there is a greatly- 

 increased quantity of biotite, which has segregated into layers giving a 

 banded appearance to the rock. 



A considerable amount of muscovite has also been dcA^eloped. and a few 

 small isotropic crystals of garnet are present. 



III. Geneeal Summaey and Conclusion &i. 



The foregoing paper, which is a third contribution to a series by 

 the present writer dealing with the geology of Peru and a part of 

 Bolivia, illustrates by means of a horizontal section across the 

 Cordillera the structural features of the country between the port 

 of Callao and the Rio Perene. 



The section commences at the Island of San Lorenzo, which is 

 formed of westward-dipping strata of Lower Cretaceous (Neo- 

 coniian) age, consisting chiefly of shales and quartzites of shallow- 

 water or littoral origin containing numerous plant-remains. They 

 here form one limb of an anticlinal fold, of which the corresponding 

 eastern limb is met with on the mainland between Callao and 

 Lima. Although igneous rocks of contemporaneous origin are 

 rare, subsequent intrusions in the form of d^^kes and sills of augite- 

 .andesite are of common occurrence. 



