14 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April I914, 



Plagioclase, in hypidioniorphic crystals, fairly fresh, but with 

 a faint pinkish colour, due to abundant dust-like ultra-microscopic 

 inclusions of iron -ores arranged parallel to the vertical axis. 

 Twinning according to polysynthetic and pericline laws. The 

 extinction-angles indicate acid labradorite. 



Pyroxenes: Monoclinic, diallage, of a faint greenish colour, 

 without definite crystal outlines ; numerous inclusions of ilmenite 

 and apatite. Rhombic, hypersthene, showing straight extinc- 

 tion, and faint pink and green pleochroism, altering into yellowish- 

 green serpentine. Both rhombic and monoclinic forms frequently 

 ■exhibit marked schillerization. 



Apatite is fairly abundant, in well-developed prisms and long 

 needles. Ilmenite is abundant in large irregular grains. Biotite 

 •occurs sparingly, as a minor accessory. Quartz is found as an 

 original constituent, the last product of consolidation, in some little 

 •quantity. 



(&) The plutonic core of the Western Cordillera. 



The erosion of the river- valleys which has brought to light the 

 Jurassic sediments has also laid bare the underlying mass of 

 plutonic rock, by which they have been penetrated and meta- 

 morphosed. This mass, which has the form of an elongated 

 batholith, with its long axis lying approximately parallel to the 

 •coast (that is, north-west and south-east), may be regarded as the 

 ■deep-seated core of the "Western Cordillera. In the district now 

 described the actual date of the intrusion can only be determined 

 us post- Jurassic, though farther north in Peru similar (and possibly 

 identical) plutonic rocks penetrate fossiliferous Cretaceous beds, and 

 -are, therefore, probably of Tertiary age, doubtless representing a 

 comparatively late phase in the development of the mountain-ranges 

 of the Andes. 



In the Llutah Valley above Molino, the plutonic core is first seen 

 near Tiiiares, and is well exposed between Cata and Palmani. It 

 is again met with at the bottom of the gorge at Jamiraya, although 

 no longer visible where the river is once more accessible at Aneblo- 

 calla, a few miles away to the east. 



The composition of the rock varies considerably in different 

 localities, which probably accounts for the different views concerning 

 its nature expressed by various authors. A. d'Orbigny regarded it as 

 a granite of very early date ; while Forbes, noting the metamorphism 

 •of the Jurassic sediments, classed it as a pre- Cretaceous diorite. 



A certain amount of differentiation appears to have taken place 

 previous to the intrusion, for it is possible to distinguish a more 

 basic type of rock representing the portion of the magma that 

 consolidated first, and a more acid later type which frequently 

 includes rounded fragments of the former. In many cases these 

 ■* cognate xenoliths ' are so numerous as to alter completely the 

 appearance of the rock. Above Cata, in fact, the basic xenoliths 

 formed about a quarter of its bulk. 



The latest product of consolidation is represented by a series of 



