part 1] THROUGH THE ANDES OF PERU AND BOLIVIA. 15 



purpose : for example, the plural description of a single dyke, met 

 with in more than one cutting, would not affect the determination 

 of the relative ages of intrusion and country-rock, provided that 

 the separate exposures were noted as being of the same type. 



The earliest rock of the sequence is first seen at kilometre 124, 

 in the form of a granodiorite (A 76) almost identical in character 

 and appearance with that previously described from the foot-hills of 

 (Jachendo (A 95), and in all probability belonging to the same 

 phase of intrusion. When this rock is traced up the line, however, 

 it is found to undergo a remarkable change, which on microscopic 

 examination is seen to be due to acidification and entire recrystal- 

 lization of its constituent minerals. 



Quartz, of a faintly pink colour, becomes increasingly abundant, 

 the felspars assume a distinctly green tint, and biotite is extensively 

 developed in little well-defined clusters, replacing to a large extent 

 the hornblende as the dominant ferromagnesian element. The 

 relative proportion of the two minerals, however, is a very inconstant 

 character, varying in the most pronounced manner often in the 

 space of a few inches. 



Large basic patches composed almost entirely of hornblende are 

 of frequent occurrence. These appear to represent xenoliths of 

 cognate origin, such as occur elsewhere in the unaltered rock, and 

 their somewhat obscure outline is probably due to a certain amount 

 of admixture during the recrystallization which they have under- 

 gone in common with the surrounding mass. 



Epidote is everywhere abundant, both disseminated throughout 

 the rock and associated with quartz in the form of epidosite-veins ; 

 as a rule these seem to have been formed in pre-existing fissures by 

 a process of leaching from the country-rock, though in some cases 

 the} r have the appearance of original igneous injections. 



No definite line of demarcation between this abnormal rock and 

 the normal granodiorite could be determined, and it is thought to 

 be a hybrid rock or a mixture of two distinct types, in which the 

 effects of dynamic metamorphism are superimposed on those of 

 contact-metamorphism. Its origin is discussed later. 



With the further ascent of the line to Quishuarani, the change 

 in the granodiorite is still more remarkable. 



The rock assumes a banded character, at first somewhat indistinct 

 With further development of biotite, then becoming increasingly 

 pronounced, until at kilometre 145 it is seen to be completely 

 crushed into a true gneiss of extremely fine grain (A 81), in which 

 the clusters of biotite have been drawn out into thread-like lines, 

 only distinguishable by their dark colour. 



The forces which produced so marked a structural change in 

 this rock must have been considerable, and as they seem to have had 

 no effect on the other rocks of the complex, we might safely regard 

 the latter, even in the absence of further evidence, as being of 

 subsequent origin, for there is no reason- to suppose that they were 

 of a more resistant nature than the granodiorite. When the 



