crests, and not even always on the middle of the ridge, but sometimes at its extremity: such is Pichincha, situated between the Pacific and the city of Quito, and which acquired celebrity in connection with Bouguer's earliest barometric formulæ, and such are the volcanos which rise in the elevated Steppe de los Pastos, itself ten thousand (10657 English) feet high. All these summits, which are of various shapes, consist of trachyte, formerly called Trap-*porphyry: a granular vesicular rock composed of different kinds of feldspar (Labradorite, Oligoklase, and Albite), augite, hornblende, and sometimes interspersed mica, and even quartz. In cases where the evidence of the first outburst or eruption, or I might say where the ancient structure or scaffolding remain entire, the isolated conical mount is surrounded by an amphitheatre or lofty circular rampart of rocky strata superimposed upon each other. Such walls or ring-formed ramparts are called "craters of elevation," a great and important phenomenon, concerning which a memorable treatise was presented to our Academy five years ago (i. e. in 1818), by the first geologist of our time, Leopold von Buch, from whose writings I have borrowed several of the views contained in the present discussion.
Volcanos which communicate with the atmosphere through permanent openings, conical basaltic hills, and craterless trachytic domes, sometimes as low as Sarcouy, sometimes as lofty as the Chimborazo, form various groups. Comparative geography shows us sometimes small clusters or distinct systems of mountains, with craters and lava-currents in the Canaries and the Azores, and without craters and without
