part 1] THEOTJGH THE ANDES OP PERU AND BOLIVIA. 21 



to contact-metamorphism alone, the secondary felspar was found to 

 differ but slightly from the original andesine, being in fact slightly 

 more basic. The change is accompanied by removal of the inclu- 

 sions, and hy a gradual obliteration of the twin-lamellation. 



One of the last stages of metamorphism before the final granu- 

 lation takes place, is marked by a pronounced sericitization of the 

 alkaline felspars, and thus it seems that this must also be attributed 

 to dynamic agenc}^. 



The following descriptions give a more detailed account of some 

 ■of the leading types : — 



(A 76a) Xenolith in granodiorite. (Kilometre 129.) (PI. II. fig-. 4.) 



A specimen selected from one of the basic patches which appear to be 

 included xenoliths of an earlier-formed rock. A coarse-grained, holo- 

 crystalline, melanocratic rock composed largely of ferromagnesian 

 minerals (hornblende and biotite). 

 Microscopic characters : — The rock has the appearance of having been 

 entirely recrystallized : the three chief constituents — plagioclase, horn- 

 blende, and biotite — being remarkably fresh and free from alteration- 

 products. No effects that can be attributed to dynamic metamorphism 

 are noticeable, and it is probable that the recrystallization was the 

 direct result of incorporation by the granodiorite magma. 



The felspar, near andesine, occurs in the form of relatively- large clear 

 crystals, characterized by abundant inclusions of quadrate grains of 

 magnetite and minute prisms and rounded grains of hornblende. Twin- 

 lamellation is often ill-defined. 



A strongly-pleochroic deep-green hornblende is abundant; it tends to 

 show idiomorphic contours to the biotite, which is also present in con- 

 siderable quantity. Both minerals seem to have recrystallized in common 

 with the felspar. 



Stout prisms and needles of a.patite are fairly numerous, and epidote, 

 sphene, and calcite occur sparingly. 



Magnetite is chiefly confined to inclusions in the felspar. 



(A 76) Metamorphosed granodiorite. (Kilometre 129.) A specimen 

 showing the result of invasion and metamorphism by an acid magma. 



In a hand- specimen this rock is almost identical with the granodiorite 

 of Posco (A 95). 

 Microscopic characters : — The rock consists essentially of quartz, felspar, 

 and hornblende, with secondary biotite and epidote, and accessory sphene, 

 apatite, and magnetite. 



The ragged outlines of the felspar and hornblende show clearly the 

 effects of corrosion by the invading magma, which has consolidated in 

 the form of a coarse quartz-mosaic. 



Hornblende is abundant, and has the appearance of having been re- 

 crystallized. It is intensely pleochroic, with colours ranging from yellow 

 to a deep bottle-green. The crystals are often fractured, the spaces 

 between the separate fragments being filled with secondary quartz and 

 felspar. 



Biotite is not abundant ; it is here seen in process of formation from 

 large patches of green chlorite crowded with magnetite and epidote, 

 which are being produced at the expense of the hornblende. 



Clear andesine-crystals with inclusions of magnetite and hornblende, 

 as in A 76 a, are occasionally noticeable ; but for the greater part the fel- 

 spar appears to be in process of recrystallization. The secondary felspar 

 forms a fine gramdar mosaic similar in appearance to micropegmatite — in 

 fact, except for an occasional indication of twinning, it is not easy to 



