The Vilcapampa batholith has two highly contrasted borders, whether we consider the degree of metamorphism of the country rock, the definition of the border, or the resulting topographic forms. On the northeastern ridge at Colpani the contact is so sharp that the outstretched arms in some places embrace typical granite on the one hand and almost unaltered shales and slates on the other. Inclusions or xenoliths of shale are common, however, ten and fifteen miles distant, though they are prominent features in a belt only a few miles wide. The lack of more intense contact effects is a little remarkable in view of the altered character of the inclusions, all of which are crystalline in contrast to the fissile shales from which they are chiefly derived. Inclusions within a few inches of the border fall into a separate class, since they show in general but trifling alteration and preserve their original cleavage plains. It appears that the depth of the intrusion must have been relatively slight or the intrusion sudden, or both shallow and sudden, conditions which produce a narrow zone of metamorphosed material and a sharp contact.
Fig. 142—The deformative effects of the Vilcapampa intrusion on the northeastern border of the Cordillera. The deformed strata are heavy-bedded sandstones and shales and the igneous rocks are chiefly granites with bordering porphyries. Looking northwest near Puquiura. For conditions near Pampaconas, looking in the opposite direction, see Fig. [141]. For conditions on the other side of the Cordillera, see Fig. [146].
The relation between shale and granite at Colpani is shown in [143] . Projections of granite extend several feet into the shale and slate and generally end in blunt barbs or knobs. In a few places there is an intimate mixture of irregular slivers and blocks of crystallized sediments in a granitic groundmass, with sharp lines of demarcation between igneous and included material. The contact is vertical for at least several miles. It is probable that other localities on the contact exhibit much greater modification and invasion of the weak shales and slates, but at Colpani the phenomena are both simple and restricted in development.