Ap'ennines, THE, a prolongation of the Alps, forming the "backbone of Italy," are perhaps the most recently formed mountains in Italy. The average height of the mountains composing the range is about 4000 feet, and nowhere do they reach the limits of perpetual snow, though some summits exceed 9000 feet in height. Monte Corno, the highest peak, has an altitude of 9580 feet. On the highest summit of the Northern Apennines, Monte Cimone (7110 feet) is a meteorological observatory. These mountains consist almost entirely of limestone rocks, and are exceedingly rich in the finest marbles. On the south slopes volcanic masses are not uncommon, Mount Vesuvius, the only active volcano on the continent of Europe, being an instance. The lower slopes are well clothed with vegetation; the summits are sterile and bare. Thirteen passes pierce the Apennines.