part 1] THROUGH THE AXDES OF PERU AND BOLIVIA^ 25 



small basic patches, which are obviously fragments of the latter 

 rock plucked off and partly absorbed. These xenoliths have been 

 almost entirely recrystallized, with the development of relatively- 

 large phenocrysts of a stronglv-pleochroic deep-green hornblende ; 

 traces of original augite with multiple twinning, however, are 

 still visible. 



The diorite along the margin of contact appears to have been 

 completely fused, and has assumed a finely-banded structure due to 

 fluxion. 



The subsequent recrystallization of the component minerals 

 throws considerable light on the origin of the xenoliths in the 

 granodiorite of the first phase, the marked similarity of the 

 resultant products of metamorphism in the two eases being a strong 

 argument in favour of the theory that the latter were derived from 

 a deep-seated dioritic magma, although this does not appear to have 

 been drawn upon extensively until a later period in the history of 

 the complex, as exemplified by the rocks of the second phase 



In the case of the earlier xenoliths, owing no doubt to a higher 

 degree of viscosity, there seems to have been but little fiuxional 

 movement, and as a result, the growth of individual crystals was 

 unrestricted. In the present case, however, the diorite was rendered 

 completely molten by the invading granite, and movement taking 

 place during consolidation, the crystals were broken up as they 

 formed, and in consequence never attained large dimensions. 



There is direct and unmistakable evidence that the secondary 

 green hornblende has here been derived from pyroxene, even though 

 the latter had in some cases been uralitized previous to recrystal- 

 lization. It is not unreasonable, then, to conclude that the xenoliths 

 in the granodiorite also had their origin in a pyroxene-bearing 

 diorite : for, notwithstanding the difference in texture of the two 

 rocks, their mineralogical characters are almost identical ; the 

 resemblance being especially well shown in the case of the glassy- 

 clear recrystallized andesine, with its abundant minute inclusions 

 of hornblende, biotite, and magnetite (see PI. II, fig. 4). 



(A 85) Adamellite. Tiabaya (kilometre 161). This rock consists essen- 

 tially of quartz, orthoclase, plagioclase, hornblende, and biotite, with 

 accessory sphene, magnetite, apatite, and zircon. 



Clear quartz is abundant. 



Orthoclase occurs in large plates surrounding idiomorphic crystals 

 of plagioclase (andesine-acid labradorite), showing albite -twinning and 

 zonary structure. The two forms are present in about equal proportions, 

 and are remarkably clear and fresh. 



The ferromagnesian minerals, which are quite subordinate to the 

 felspars, comprise idiomorphic green hornblende, twinned parallel to 100, 

 and brown biotite showing alteration into chlorite. 



Magnetite and sphene are of fairly-common occurrence, the latter in 

 well -developed crystals. 



Apatite and zircon occur sparingly. 



Associated with the adamellite, and veining both this rock and 

 the older diorites, are a few fine-grained acid dykes. These consist 

 of quartz and turbid orthoclase, often in micropegmatitic inter- 



