Metamorphic Rocks of the Eastern Andes near Cuenca, Ecuador

Extract

The object of the present paper is to describe certain metamorphic rocks which occur in the eastern cordillera of the Ecuadorian Andes, immediately to the east of the interandine basin of Cuenca (Fig. 1). The latter region contains a series of late Tertiary volcanic breccias, followed by hard white shales which are overlain in their turn, and possibly in part replaced laterally by the massive, white, saccharoidal and tuffaceous sandstones of Azogues, in the composition of which rearranged volcanic ash and detritus play a large part (Fig. 2). These beds, originally described collectively as the Azogues Sandstone by Wolf (1),¹ were considered to be of Wealden age upon the evidence of a meagre lacustrine fauna discovered by the same author, and subsequently described by Dr. Geinitz, of Dresden, which were found in the more shaly sandstones of the group. A more extensive collection of fossils made by the present writers from the same beds and localities was submitted to Dr. W. B. Marshall, of the United States National Museum, who reports that it consists largely of new genera and species, and that the age cannot be later than the Pliocene, and it may be earlier (2). These beds are thrown generally into a series of relatively gentle folds, having a north-south trend, but occasionally they are found in a more highly disturbed condition as, for example, on the Biblian-Azogues road, where they are sometimes seen to be vertical. The strike of the folding shows that it is a product of pressure from the same direction as that which accompanied the main Andean uplift.