24 MR. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [vol. lxxvi, 



have been fractured and sheared into irregular fragments, still pre- 

 serving their optical continuity, though separated by patches of 

 clear plagioclase and quartz. This feature, in fact, is even visible 

 in a hand-specimen, the plates of biotite giving a distinctive lustre- 

 mottling to the rock. 



(A 77) Augite-diorite. (Kil. 132.) This rock consists essentially of 

 plagioclase, a colourless augite and biotite, with a little quartz and 

 orthoclase and accessory magnetite, apatite, epidote, and chlorite. 



The plagioclase (medium labradorite) is clear and fresh, showing 

 twinning on the albite and pericline laws. It is in considerable excess of 

 the orthoclase, to which it is idiomorphic. 



Chestnut-brown biotite and a pale-green or colourless augite are 

 present in roughly equal proportions, the former showing alteration into 

 chlorite and epidote. The augite occurs in the form of irregularly-bounded 

 crystals, showing multiple twinning and incipient alteration into a fibrous 

 uralitic hornblende. 



A small amount of interstitial quartz, magnetite in some abundance, 

 and a little apatite are also present. 



(A 84) Saussuritized diorite. Huaico (kilometre 156). This rock is 

 a modification of the foregoing, in which the pyroxene has been almost 

 completely converted into a bluish-green fibrous actinolite, frequently 

 showing a fringe of secondary biotite. 



A deep-brown biotite also occurs, in the form of large irregular plates 

 usually surrounding magnetite. 



Although a considerable amount of plagioclase is clear and unaltered, 

 most of the larger crystals are opaque with decomposition-products : 

 these consist chiefly of minute highly-refracting prisms and granules, 

 which between crossed nicols show the bright interference-colours of 

 epidote and the deep blue of zoisite, the change therefore appearing to be 

 one of saussuritization. It is difficult to determine the nature of the 

 original felspar, but the extinction-angles when visible suggest that it 

 was less basic than the unaltered crystals, which are acid labradorite. 



The general appearance of the section, however, makes it not im- 

 probable that we are dealing here with a mixed rock, and the fresh felspar 

 and most of the quartz, which occurs in some abundance, may have been 

 derived from an extraneous source. 



The third and final phase in the sequence was marked by 

 the intrusion of a granitic magma of acid character, probably 

 derived from the same deep-seated reservoir as that which furnished 

 the acid magma which invaded the granodiorite. Its intrusion, 

 however, is obviously of much later date, for it has nowhere been 

 affected by the dynamic stresses which produced so marked a 

 structural change in the latter rock. It is seen, moreover, to be 

 posterior to the dioritic rocks of the second phase, for these show 

 clearly the effects of contact-metamorphism at the points of 

 junction. 



Th'. 1 - typical rock of this granitic phase is a coarse-grained grey 

 adamellite, very similar in appearance to the rock previously 

 mentioned as cropping out in the desert near Huagri. It is exten- 

 sively quarried, for building-purposes, in the neighbourhood of 

 Tingo, and has been used in the construction of the mole at 

 Mollendo. Near its junction with the diorite it contains numerous 



