A broad chain of lofty mountains running from the Bay of Biscay, 276 m. eastwards, to the Mediterranean, form the boundary between France and Spain. They are highest in the centre, Mount Maladetta reaching 11,168 ft. The snow-line is about 8000 or 9000 ft., and there are glaciers on the French side. Valleys run up either side, ending in precipitous "pot-holes," with great regularity. The passes are very dangerous from wind and snow storms. The streams to the N. feed the Adour and Garonne; those to the S., the Ebro and Douro. Vegetation in the W. is European, in the E. sub-tropical. Minerals are few, though both iron and coal are worked. The basis of the system is granite with limestone strata superimposed.