Academy of Sciences in Paris. 373 



does not appear to exist in the Pyrenees. The masses of granite 

 form, as it were, large islands in the chain, which do not agree 

 either with themselves or with the geographical axis. The western 

 region of the French Pyrenees contains tracts of aphite, having 

 nearly the same direction as the chain from east to west ; but they 

 do not exist in the eastern region. The French Pyrenees, particu- 

 larly those valleys through which flow the streams tributary to the 

 Garonne and the Adour, contain a number of oblique ridges, which, 

 as well as their strata, are directed towards W. N. W., and some- 

 times even towards the N. W., whence has arisen the great error 

 of applying this law to the whole chain, and supposing the direction 

 of strata to be parallel to the Pyrenean axis ; whereas, in fact, that 

 axis and the strata directed towards the W. N. W., cut each other in 

 an angle of at least fifteen degrees. This error might have been 

 avoided by observing that the rule above alluded to is by no means 

 a general one ; some of the ridges of the Pyrenees are directed 

 towards the W.S.W., and the strata follow the same direction; that of 

 Canigou, for instance, on the summit of which are seen gneiss and 

 micaceous schistus, having the same direction as the protuberance of 

 which they form the pinnacle. A few strata are also found having 

 the same direction as the total chain from east to west. The sinu- 

 osities of the small ridges, their obliquity with respect to the cen- 

 tral axis, and their junctions in one binuous summit, prove that 

 the Pyrenean chain was thrown up at different epochs. This fact, 

 which is derived from the irregularities of detail in the central crest, 

 is confirmed by the relation of its great and principal divisions. In- 

 dependently of the small chains which may be traced on the two 

 declivities, there are three principal and distinct chains, which con- 

 tribute to form the long summit of the Pyrenees. The ridge or chain 

 which overhangs the eastern region follows the direction from 

 E. N. E. to W. S. W. It extends from the plain of Roussillon to 

 that of Catalonia, skirting in France the right bank of the Est, and 

 in Spain the left bank of the Segre. Its numerous summits attain 

 heights of 1400 to 1500 toises; the Perigmal of Cerdagna is the 

 most elevated point. The central crest, which is cut by this 

 ridge between Mont Louis and Prati de Mallo, is much inferior to 

 it in height. The valleys of the Est and the Segre form, at the foot 

 of this chain, the only longitudinal section of this nature which 

 exists in the whole Pyrenean chain. The great basin of Cerdagna 

 (the largest in the Pyrenees) occupies the culminating point of the 

 double valley, which appears to be situated in the linear direction of 

 the summit, about 600 toises below the highest point. To the 

 N. W. of this basin rises the second ridge, directed towards the 

 W. J N. ; its height is nearly 1500 toises, at the source of the eastern 

 Arriege, and exceeds that elevation in the region of the western 

 Arriege. It extends into the region of the Salat, and to the first 

 branches of the valley of Etran ; then gradually diminishes in 

 height, and is lost amid the mountains of the French declivity. 



