Vol. 70.] THKOTJGH THE ANDES OF PERU AND BOLIVIA. 17 



along the site of folded geosynclines, and lie with their long axes 

 parallel to the axes of the mountain-ranges. One of the most 

 prolific writers on this subject is Dr. E-. A. Daly, who considers that 

 the replacement has been caused partly by digestion and solution 

 along the contact of magma and country-rock, but chiefly by the 

 mechanical process of stoping, which is the only hypothesis that 

 accounts for the apparent lack of chemical relation between the 

 granite and the country-rock, impossible to explain by a doctrine 

 of simple assimilation. 



Such a mode of origin for this batholithic core seems to be in 

 accordance with the observed facts ; but, since it is elongated 

 parallel to the general trend of the Cordillera, we are dealing 

 here with comparatively-limited exposures afforded by the Llutah 

 and Palca Rivers, which cut across the strike. In } r oung steep- 

 sided valleys of this nature, however, where the contact with the 

 overlying rocks is to a great extent obscured by screes, much 

 evidence is not forthcoming in support or disproof of any theory 

 as to the relative importance of the physical process of stoping or 

 the chemical process of assimilation. Both have probably had 

 effect, and, in describing the core of the Western Cordillera as a 

 batholith, I have interpreted the term in this double sense, thereby 

 expressing my views as to its probable mode of origin. The mass 

 is dome-shaped in transverse section, widening downwards without 

 any visible floor, and cuts across the overlying Mesozoic deposits 

 irrespective of their dip. The process of intrusion appears to have 

 been essentially one of replacement rather than displacement, and 

 I see no reason to criticize Dr. Daly's theory that this may have 

 been in great part effected by actual mechanical stoping. 



One of the most frequently mentioned objections to the theory 

 of batholithic replacement is that, as a rule, there is little or no 

 evidence of solution by the granitic magma. In the present case, 

 such evidence is not wholly lacking. A few hundred yards above 

 Cata on the south side of the river, a peculiar rock is met with, 

 apparently passing downwards into the ordinary acid plutonic 

 type (A n ). This rock has, at first sight, the appearance of a 

 compact quartzite ; but, in microscopic section (A 10 ; PI. VII, 

 figs. 1 & 2), it is seen to be of igneous origin and composed of a 

 granular mosaic of quartz and felspar (chiefly orthoclase), with 

 abundant irregular plates of hornblende and biotite, com- 

 pletely devoid of crystal outline and showing pronounced corrosion. 

 It appears to represent the typical basic granodiorite, invaded and 

 partly assimilated by a highly-acid magma. The latter has con- 

 solidated as a pegmatitic intergrowth of quartz and orthoclase 

 in the actual process of dissolving l the ferromagnesian elements 

 of the original rock. That the phenomenon is one of corrosion 

 and not of simultaneous crystallization is particularly noticeable 



1 Instances of a similar process of solution by residual quartz were figured 

 and described from the Gilgit granite in the Himalayas by C. A. McMahon, 

 Q. J. G. S. vol. lvi (1900) p. 366 & pi. xxiii. 



Q. J. G. S. No. 277. c 



