Home >> Encyclopedia-britannica-volume-18-plants-raymund-of-tripoli >> Purification to Rabban Bar Sauma >> Pyrenees_P1

Pyrenees

zone, cretaceous, eocene, central, jurassic, crest and lower

Page: 1 2

PYRENEES, a range of mountains in south-west Europe (Span. Pirineos, Fr. Pyrénees), separating the Iberian Peninsula from France, and extending for about 240 m., from the Bay of Biscay to Cape Creus, or, if only the main crest of the range be considered, to Cape Cerbere, on the Mediterranean Sea. The main crest constitutes the Franco-Spanish frontier; except in the case of the valley of Aran, which belongs orographically to France but politically to Spain. The Pyrenees are conventionally divided into three sections, the central, the Atlantic or western, and the Medi terranean or eastern. The central Pyrenees extend eastward from Canfranc to the valley of Aran, and include the highest summits of the whole chain, Aneto or Pic de Nethou (11,168 ft.), in the Mala detta ridge, Posets (11,047 f t.), and Mont Perdu or Monte Perdido (10,997 ft.). In the Atlantic Pyrenees the average altitude grad ually diminishes westward; while in the eastern Pyrenees, except for one break at the eastern end of the Pyrenees Ariegeoises, the mean elevation is maintained, till a rather sudden decline occurs in the extreme eastern portion, the Alberes. This threefold division is only valid so far as the elevation of the Pyrenean chain is concerned, and does not represent its geological structure or general configuration. The southern versant of the mountains is the more important. It is recognized that the range must be regarded as an elevated part of the earth's crust, the culminating portion of which is composed of a series of chains, which do not coincide with the watershed, but cross it obliquely. Maps by Schrader and de Margerie (Ann. du Club Alpin francais, 1891 and 1892) show the orderly arrangement of these chains. The primitive formations of the range are shown to be almost all continued diagonally on the Spanish side, and the central ridge thus presents the appearance of a series of wrinkles with an inclination (from north-west to south-east) greater than that of the chain as a whole. Other less pronounced wrinkles run from south-west to north-east and intersect the former series at certain points, so that it is by alternate digressions from one to the other series that the irregular crest of the Pyrenees acquires its general direction. Far from having impressed its own direction

on the orientation of the chain at large, this crest is merely the resultant of secondary agencies by which the primitive mass has been eroded and lessened in bulk, and though its importance from a hydrographic point of view is still considerable, its geological significance is practically nil.

Geology.—The Pyrenees are divided into a number of longi tudinal zones. The central zone (Central Massifs) consists of Primary rocks, Archaean, Cambrian, Ordovician, Silurian, Devonian and Lower Carboniferous (Dinantian) together with great masses of granite. It forms most of the higher summits, but west of the Pic d'Anie it disappears beneath an unconformable covering of Cretaceous deposits. On the French side the central zone is followed by ( I) the zone of Ariege, consisting of Lower Cre taceous and Jurassic beds, together with granitic masses; (2) the zone of the Petites Pyrenees, Upper Cretaceous and Eocene; a thin outcrop of Jurassic ; (2) the zone of Aragon, Eocene; and Primary rocks. On the Spanish side, from north to south, are (I) the zone of Mont Perdu, Upper Cretaceous and Eocene with a thin outcrop of Jurassic; (2) the zone of Aragon, Eocene; and (3) the zone of the Sierras, Trias, Cretaceous and Eocene. Al though the number of zones is the same on the two flanks, they do not correspond. The zone of the Corbieres has no equivalent in Spain, while in France there is no definite zone of Eocene like that of Aragon. The zone of the Petites Pyrenees, however, is clearly homologous with that of the Sierras. On the northern side granitic masses occur in the zone of Ariège amongst the Jurassic and Lower Cretaceous beds. On the southern side they are not found except in the axial zone, and the Jurassic and Lower Cretaceous deposits are reduced to a narrow band. In spite of these differences between the two flanks, the structure is to some extent symmetrical.

Page: 1 2