Notes on the Pyrenees. 499 



logical tract; and, whatever may have been the agents which 

 have produced the contrasted configurations on this tract, 

 still the phenomena of internal structure, or the variety in 

 external characters, will always afford physical indices of the 

 nature of these influences, while, by a study of any one of them 

 alone, we may be led, from the want of proper indications, 

 into the widest field of hypothesis. The deposition of allu- 

 vial tracts by water, the piling up of mountains of sancj by 

 the winds, the formation of basaltic columns by volcanoes, 

 the uprising of forests of islands, whose architects are minute 

 and almost invisible, finally cementing together to form con- 

 tinents, are so many striking and well known features in geo- 

 logy; but the power that consolidated, or the hand that 

 hewed out, the giant forms that adorn the surface of the earth 

 has not yet been felt by man. 



Besides the mountains forming the principal chain, there 

 occur in the Pyrenees many others united by a common crest, 

 and forming chains running sometimes in a direction perpen- 

 dicular to that of the chain generally denominated transverse 

 or divergent ; others follow a course which is parallel to that 

 of the principal crest, and are thus lateral or parallel chains. 

 While the termination of the transverse chains generally takes 

 place in the plains, or in the meeting of two valleys, that of 

 the parallel or lateral chains most frequently occurs in the 

 larger transverse valleys of the chain. The most striking dif- 

 ferences between the structure of the transverse and principal 

 chains in the Pyrenees occur in the Maladetta, where, while 

 considerations founded on the phenomena of valleys, and on 

 the physical distribution of the waters, mark the transition 

 rocks as forming the principal crest, granitic rocks strike out 

 in a south-westerly direction, attaining an elevation of 1787 

 toises. * The chain of Mont Perdu, perpendicular to the 

 crest, is also transverse to the limestone chain ; and thus the 

 Ara river courses parallel to the streams supplying the Cinca 

 river. With these two exceptions, and the ridge of alpine 

 limestone taking its departure from Mont Aistaince (and the 

 accidents they present mark their difference from other trans- 

 verse chains), all the rest that take their departure from the 

 principal crest are similar in structure to the rock of which 

 they constitute branches.f 



* The French toise is, according to General Roy, equal to 1*06575 

 English fathom. 



f The memoir of M. Reboul, read to the Academy of Sciences in 

 1788, established that the calcareous beds of the Marbore and Mont Perdu 

 lie every where on granite or argillaceous schists, or on intermediary (tran- 

 sition) siliceous rocks. 



