Notes on the Pyrenees. 4.99 
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 sand by 
the winds, the formation 6f basaltic columns by volcanoes, 
the uprising of forests of islands, whose ar chitects are minute 
and almost invisible, finally cementing together to form con- 
tinents, are so many striking and well know 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 
ina couthasie ly 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. 
* The French toise is, according to General Roy, equal to 1:06575 
English fathom. 
+ The memoir of M. Reboul, read to the Academy of Sciences in 
1788, established that the calcareous beds of the Marboré and Mont Perdu 
lie every where on granite or argillaceous schists, or on intermediary (tran- 
sition) siliceous rocks. 
