Geographical Collections. 205 



which, alternating for short spaces with the former, continues to nearly the wes- 

 tern oceanic extremity of the chain. 



The crystalline rocks are not thus found constituting the greater portion of the 

 crest, nor do they in those points attain an elevation much exceeding that of the 

 transition or secondary series. The alpine limestone on the north-east of the 

 chain, constitutes a series of parallel chains, continued to the Montagne des Cor- 

 bieres, where it is succeeded by clay-slate and greywacke, by which the crest is 

 continued, while it forms a series of transverse chains, met with again at the ex- 

 tremity of the transition rocks. The band of the latter reposing on granite moun. 

 tains, re-appears on the other side, to form the crest of the principal chain, and 

 by this disposition, the Pyrenees represent in their eastern portions three different 

 chains, in which the band of transition rocks, forming the line of the principal 

 crest, predominates over the crystalline series, lying at some distance to the north- 

 east, or in the direction from which the waters should have effected the degrada- 

 tion of the first series. 



In all cases, if we proceed by describing the variety in constitution or of com- 

 bination in each separate protuberance, to give an idea of the geognostic structure 

 of a chain of hills or mountains, we shall arrive at but an imperfect notion, even 

 with the most minute details. 



Taking, however, in view the physical characters of the range with the general 

 principles of its internal constitution, we can, by tracing the elements of the dis- 

 position and accidents of the former in the departition and developement of its 

 mineral constituents, ally in our description phenomena which are never separated 

 in nature, and deduce some general notions on the origin of the mineral masses ; 

 for the variety of which the same mineral constituents are capable of assuming in 

 their combination, renders it both a tedious and useless task to pursue such in- 

 vestigations to their ultimate point. But the departition which the same minerals 

 assume, with respect to their geognostic superposition, or to their geographical 

 distribution, is of the utmost importance in deductions on the physical characters 

 of a geological tract; and whatever may have been the agents which may have 

 produced the contrasted configurations on this tract, still the phenomena of inter- 

 nal structure, or the variety in external characters, will always afford physical in- 

 dices of the nature of these influences ; while by a study of any one of these alone, 

 we may be led, from the want of proper indications, into the widest field of hypo- 

 thesis. The deposition of alluvial tracts by water, the piling up of mountains of 

 sand by the wind, the formation of basaltic columns by volcanoes, the rising up 

 of forests of islands, whose architects are minute and almost invisible, finally ce- 

 menting together to form extensive continents, are so many striking and well 

 known features in geology ; but the power that consolidates, 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 crest, there occur in the Pyrenees 

 many others united by a common crest, and forming chains, running sometimes 

 in a direction perpendicular 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 trans- 

 verse chains generally takes place in the plains, or in the meeting of two vallies, 

 that of the parallel or lateral chains takes place in the vallies of the chain. The 

 most striking differences between the structure of the transverse and principal 

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

 founded on the phenomena of vallies, and on the physical distribution of the wa- 

 ters, mark the transition rocks as forming the principal crest, granite rocks 

 strike out in a south-westerly direction, attaining an elevation of 178? 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 



• A toise, according to General Roy, is equal to 1.06575 English fathoms. 



