At Pasaje, on the western side of the Apurimac, the Carboniferous again appears resting upon the old schists described on p. [236]. It is steeply upturned, in places vertical, is highly conglomeratic, and in a belt a half-mile wide it forms true badlands topography. It is succeeded by evenly bedded sandstones of fine and coarse composition in alternate beds, then follow shales and sandstones and finally the enormous beds of limestone that characterize the series. The structure is on the whole relatively simple in this region, the character and attitude of the beds indicating their accumulation in a nearly horizontal position. Since the basal conglomerate contains only pebbles and stones derived from the subjacent schists and does not contain granites like those in the Cordillera Vilcapampa batholith on the east it is concluded that the batholithic invasion was accompanied by the compression and tilting of the Carboniferous beds and that the batholith itself is post-Carboniferous. From the ridge summits above Huascatay and in the deep valleys thereabouts the Carboniferous strata may be seen to extend far toward the west, and also to have great extent north and south. Because of their dissected, bare, and, therefore, well-exposed condition they present exceptional opportunities for the study of Carboniferous geology in central Peru.
Fig. 160—The deformative effects of the granite intrusion of the Cordillera Vilcapampa are here shown as transmitted through ancient schists to the overlying conglomerates, sandstones, and limestones of Carboniferous age, in the Apurimac Valley at Pasaje.
Carboniferous strata again appear at Puquiura, Vilcapampa, and Pampaconas. They are sharply upturned against the Vilcapampa batholith and associated volcanic material, chiefly basalt, porphyry, and various tuffs and related breccias. The Carboniferous beds are here more arenaceous, consisting chiefly of alternating beds of sandstone and shale. The lowermost beds, as at Pongo de Mainique, are dominantly marine, fossiliferous limestone beds having a thickness estimated to be over two miles.
From Huascatay westward and southward the Carboniferous is in part displaced by secondary batholiths of granite, in part cut off or crowded aside by igneous intrusions of later date, and in still larger part buried under great masses of Tertiary volcanic material. Nevertheless, it remains the dominating rock type over the whole stretch of country from Huascatay to Huancarama. In the northwestern part of the Abancay sheet its effect on the landscape may be observed in the knife-like ridge extending from west to east just above Huambo. Above Chuquibambilla it again outcrops, resting upon a thick resistant quartzite of unknown age, [162] . It is strongly developed about Huadquirca and Antabamba and, still associated with a quartzite floor, it finally disappears under the lavas of the great volcanic field on the western border of the Andes. Figs. 141 and 142 show its relation to the invading granite batholiths and [162] shows further structural features as developed about Antabamba where the great volcanic field of the Maritime Cordillera begins.