part 1] THROUGH THE A>"DES OF PERU A>~1) BOLIVIA. 19 



.and show sericitization ; but, where the granodiorite is in excess, 

 .andesine is the dominant form. A few crystals show the original 

 inclusions of magnetite and hornblende ; but more usually they 

 .are seen to be in the process of recrystallization, with the produc- 

 tion of a mosaic of secondary felspar of similar or slightly more 

 basic composition. 



Occasional deep-green crystals of hornblende are taken to 

 represent xenocrysts derived from the basic patches, suggesting 

 that the recrystallization of these had already taken place prior to 

 rthe invasion of the acid magma. 



The origin of the biotite clusters is of especial interest. Two 

 possible sources suggest themselves : — 



(1) That they are primary derivatives from the acid magma, of the nature 

 .of small glomeroporphyritic aggregates, formed in the normal order of crystal- 

 lization. (2) That they are secondary products of metamorphism. built up at 

 .the expense of other minerals. 



An examination of the altered granodiorite in the immediate 

 .neighbourhood of the acid rock supports the latter view. The 

 .characteristic green hornblende is here seen in process of con- 

 version into a yellowish-green chloritoid substance, with which are 

 .associated abundant grains of magnetite and crystals of epidote. 

 .A segregation of these coloured minerals into patches is already 

 .noticeable, which it is not easy to reconcile with a simple process 

 of contact-metamorphism alone, unless the rock has been rendered 

 sufficiently viscous for diffusion to have taken place. By re- 

 absorption of the magnetite the green chlorite becomes deep brown, 

 .and is gradually transformed into biotite. 



Where the biotite clusters are fully developed the chlorite is 

 typically absent, the magnetite has decreased considerably in quan- 

 tity, and the green hornblende has almost entirely disappeared. 



The granodioritic phase was brought to a close by the develop- 

 ment of powerful oro genie movements, which produced intense 

 .crushing in these earlier-formed rocks of the complex. The 

 .dynamic action, however, was chiefly marginal in effect, and 

 hence the most pronounced metamorphic phenomena attributable 

 to this cause are not of wide extent. 



The resultant changes which the rocks have undergone are both 

 structural and mineralogical in character. Although the former 

 .are, of course, the more obvious, the latter make their appearance 

 first, and indeed are observed before any actual deformation of the 

 rock is noticeable. It is only by tracing such changes laterally 

 from the seemingly unaltered rock into the crushed and banded 

 gneiss that the true nature of their origin becomes apparent. 



The characteristic green hornblende, for example, is then seen to 

 have been formed prior to the crushing, for obvious effects of 

 mechanical action are displayed in the breaking-up of well-defined 

 crystals along cleavage-cracks and subsequent shearing into in- 

 numerable small rhombs and prisms, groups of which, more often 

 than not, retain their optical continuity. 



c2 



