_Geology._[1]--The great longitudinal depression which lies between the eastern and the western branches of the Andes is also the boundary between the ancient rocks of the east and the Mesozoic beds which form the greater part of the west of the country. The Eastern Cordillera is composed of gneiss, mica and chlorite schist and other crystalline rocks of ancient date; the Western Cordillera, on the other hand, is formed of porphyritic eruptive rocks of Mesozoic age, together with sedimentary deposits containing Cretaceous fossils. Most of the country between the Andes and the sea is covered by Tertiary and Quaternary beds; but the range of hills which runs north-west from Guayaquil is formed of Cretaceous and porphyritic rocks similar to those of the Andes. In the intra-andine depression, between the East and West Cordilleras, recent deposits with plant remains occur near Loja, and to the north-east of Cuenca is a sandstone containing mercury ores, somewhat similar to that of Peru. Farther north nearly the whole of the depression is filled with lavas, tuffs and agglomerates, derived from the Tertiary and recent volcanoes which form the most striking feature of the Andes of Ecuador. These volcanoes are most numerous in the northern half of the country, and they stand indifferently upon the folded Mesozoic beds of the Western Cordillera (e.g. Chimborazo, Iliniza, Pichincha), the ancient rocks of the Eastern Cordillera (Altar, Tunguragua, Cotopaxi, Antisana), or the floor of the great depression between. The lavas and ashes are for the most part andesitic.