EASTERN ANDES: CORDILLERA VILCAPAMPA 223 



proach to lateral contacts, it may reasonably be inferred that they 

 represent foundered masses of country rock. Those distant from 

 present contacts are in almost all cases from a few inches to a 

 foot in diameter, while on approach to lateral contacts they are 

 in places ten to twenty feet in width, as if the smaller areas rep- 

 resented the last remnants of large inclusions engulfed in the 

 magma near the upper or roof contact. They are so thoroughly 

 injected with silica and also with typical granite magma as to 

 make their reference to the country rock less secure on petro- 

 graphical than on purely distributional grounds. 



A parallel line of evidence relates to the distribution of com- 

 plementary dikes throughout the granite. In the main mass of 

 the batholith the dikes are rather evenly distributed as to kind 

 with a slight preponderance of the dark-colored group. Near the 

 contact, however, aplitic dikes cease altogether and great num- 

 bers of melanocratic dikes appear. It may be inferred that we 

 have in this pronounced condition suggestions of strong influence 

 upon the final processes of invasion and cooling of the granite 

 magma, on the part of the country rock detached and absorbed 

 by the invading mass. It might be supposed that the indicated 

 change in the character of the complementary dikes could be 

 ascribed to possible differentiation of the granite magma whereby 

 a darker facies would be developed toward the Colpani contact. 

 It has, however, been pointed out already that the darkening of 

 the granite in this direction is intimately related to a marked in- 

 crease in the number of inclusions, leaving little doubt that the 

 thorough digestion of the smaller masses of detached shales is 

 responsible for the marked increase in the number and variety 

 of the ferro-magnesian and special contact minerals. 



Upon the southwestern border of the batholith the number of 

 aplitic dikes greatly increases. They form prominent features, 

 not only of the granite, but also of the schists, adding greatly to 

 the strong contrast between the schist of the border zone and that 

 outside the zone of metamorphism. In places in the border 

 schists, these are so numerous that one may count up to twenty 

 in a single view, and they range in size from a few inches to ten 



