Fig. 147—Overthrust folds in detail on the southwestern border of the Vilcapampa batholith near Chuquibambilla. The section is fifteen feet high. Elevation, 13,100 feet (4,000 m.). For comparison with the structural effects of the Vilcapampa intrusion on the northeast see [Fig. 142].
The granite becomes notably darker on approach to the northeastern contact near Colpani; the proportion of ferro-magnesian minerals in some cases is so large as to give a distinctly black color in sharp contrast to the nearly white granite typical of the central portion of the mass. Large masses of shale foundered in the invading magma, and upon fusion gave rise to huge black masses impregnated with quartz and in places smeared or injected with granite magma. Everywhere the granite is marked by numbers of black masses which appear at first sight to be aggregations of dark minerals normal to the granite and due to differentiation processes at the time of crystallization. It is, however, noteworthy that these increase rapidly in number on approach to the contact, until in the last half-mile they appear to grade into the shale inclusions. It may, therefore, be doubted that they are aggregations. From their universal distribution, their uniform character, and their marked increase in numbers on approach 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 represented 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 petrographical than on purely distributional grounds.
A parallel line of evidence relates to the distribution of complementary 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 numbers 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 increase 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 or fifteen feet. The greater fissility of the schists as contrasted with the shales on the opposite or eastern margin of the batholith caused them to be relatively much more passive in relation to the granite magma. They were not so much torn off and incorporated in the magma, as they were thoroughly injected and metamorphosed. Added to this is the fact that they are petrographically more closely allied to the granite than are the shales upon the northeastern contact.
CHAPTER XIV
THE COASTAL TERRACES
ALONG the entire coast of Peru are upraised and dissected terraces of marine origin. They extend from sea level to 1,500 feet above it, and are best displayed north of Mollendo and in the desert south of Payta. The following discussion relates to that portion of the coast between Mollendo and Camaná.
At the time of the development of the coastal terraces the land was in a state of temporary equilibrium, for the terraces were cut to a mature stage as indicated by the following facts: (1) the terraces have great width—from one to five and more miles; (2) their inner border is straight, or, where curves exist, they are broad and regular; (3) the terrace tops are planed off smoothly so that they now have an even gradient and an almost total absence of rock stacks or unreduced spurs; (4) the mature slopes of the Coast Range, strikingly uniform in gradient and stage of development ([Fig. 148]), are perfectly organized with respect to the inner edge of the terrace. They descend gradually to the terrace margin, showing that they were graded with respect to sea level when the sea stood at the inner edge of the highest terrace.
From the composition and even distribution of the thick-bedded Tertiary deposits of the desert east of the Coast Range, it is concluded that the precipitation of Tertiary time was greater than that of today (see p. 261). Therefore, if the present major streams reach the sea, it may also be concluded that those of an earlier period reached the sea, provided the topography indicates the perfect adjustment of streams to structure. Lacustrine sediments are absent throughout the Tertiary section. Such through-flowing streams, discharging on a stable coast, would also have mature valleys as a consequence of long uninterrupted erosion at a fixed level. The Majes river must have cut through the Coast Range at Camaná then as now. Likewise the Vitor at Quilca must have cut straight across the Coast Range. An examination of the surface leading down from the Coast Range to the upper edge of these valleys fully confirms this deduction. Flowing and well-graded slopes descend to the brink of the inner valley in each case, where they give way to the gorge walls that continue the descent to the valley floor.
Confirmatory evidence is found in the wide Majes Valley at Cantas and Aplao. (See the Aplao Quadrangle for details.) Though the observer is first impressed with the depth of the valley, its width is more impressive still. It is also clear that two periods of erosion are represented on its walls. Above Aplao the valley walls swing off to the west in a great embayment quite inexplicable on structural grounds; in fact the floor of the embayment is developed across the structure, which is here more disordered than usual. The same is true below Cantas, as seen from the trail, which drops over two scarps to get to the valley floor. The upper, widely opened valley is correlated with the latter part of the period in which were formed the mature terraces of the coast and the mature slopes bordering the larger valleys where they cross the Coast Range.