The chain of the Andes is divided into three ranges of mountains running from south to north in the transverse group or mountain-knot of Pasco and Huanuco, which shuts in the valley of Bombon between the 11th and 10th parallels of south latitude: that in the centre separates the wide fertile valley of the Upper Marañon from the still richer valley of the Huallaga, whilst the more eastern forms the barrier between the latter and the tropical valley of the Yucayali. The western chain alone reaches the limit of perpetual snow, and, if we except the Nevado of Huaylillas, in 7° 50ʹ, no mountain north of this for nearly 400 miles to the Andes of Quito arrives at the snow-line. In lat. 4° 50ʹ S. the Andes form the mountain-knot of Loxa, once celebrated for its forests, in which the cinchona or Peruvian bark was discovered. From this knot the chain divides into two great longitudinal ridges or cordilleras, in an extent of 350 miles, passing through the republic of the Equator to the mountain-group of Los Pastos in that of New Grenada. These ridges enclose a vast longitudinal valley, which, divided by the cross ridges of Assuay and Chisinche into three basins, form the valleys of Cuença, Tapia, and Quito. The plain of Cuença offers little interest; that of Tapia is magnificent; whilst the valley of Quito is one of extraordinary beauty: on either side rise a series of snow-capped peaks, celebrated in every way in the history of science, as the valley itself is in that of the aboriginal races of the New World. Here the energies of volcanic action have been studied with the greatest fruits; here, now one hundred years ago, took place that measurement of an arc of the meridian which afforded the most accurate data at the time towards the determination of the mass and form of our planet, and which has conferred eternal honour on the body with which it originated, the French Academy of Sciences; and celebrity on the names of Bouguier, La Condamine, and Godin, Don George Juan and Ulloa, who conducted it on the part of the crowns of France and of Spain.