490 



THE GEOLOGIST. 



about thirteen kilometres, or rather more than two leagues and a 

 half.^' 



In a letter, writen at Luchon, to M. Cordier, member of the French 

 Institute, M. Leymerie gives a description of an ascent of the Muladetta, 

 and of the different kinds of granite he has observed in the Pyrenees 

 and in the Haute- Garonne. The ascent, which lies entirely over 

 granite, can now be made in two days, by following the route indicated 

 in 1842 by M. de Tranqueville. The summit of the Muladetta, called 

 le Pic de 1^'ethon (the highest summit of the Pyrenees, and, ac- 

 cordiDg to Eeboul, attaining to 11,443 feet), which, from the portof 

 Venasque, looks so acute, consists in reality of a small plateau, com- 

 posed of accumulated blocks and fragments of rock very similar to the 

 accumulated fragments observed at the summit of Mont Perdu. 



The fundamental rock of the Muladetta is a small-grained granite, 

 with white feldspar and nearly black mica, which M. Cordier has for- 

 merly described with great accuracy. But the summit of the Pic de 

 i^ethon cffers a different kind of rock — a peculiar species, which may 

 perhaps be classed between granite and quartziferous porphyry. In 

 a quartz o-feld spathic cement of a dirty grey colour, the eye distin- 

 guishes, on the surface of this rock, bits of grey quartz, scarce lamellae 

 of mica, a few spots of amphibole and rectangular crystals of rose- 

 coloured orthose (feldspar), which gives a rosy tint to the rock when 

 viewed from a little distance. This last-named species of granite 

 appears to predominate in the culminating parts of the mountain, and 

 M. Leymerie supposes it to have penetrated through the former, which 

 seems to have been uplifted in the solid state, together with the tran- 

 sition strata which overlie it. 



The small-grained granite before alluded to must, therefore, have 

 played a passive part in the formation of these high mountains, whilst 

 the grey porphyroid variety has been the eruptive element. 



It is not so, however, with the porphyroid granite, bearing large 

 macled crystals of orthose, which forms the culminating portions of the 

 Pyrenees from Maupas to Claribade, and which is very characteristic in 

 the higher parts of the valley of Oo. This one, on the contrary, offers 

 also unmistakeable eruptive characters. 



After a minute investigation of these rocks, the author comes to the 

 conclusion that there are three principal sorts of granite rocks in the 

 Pyrenees and the Haute- Garonne. 



1. The small-grained granite which might be termed, according to 

 M. Leymerie, " granite Pyreneen," from its abundance in the Pyrenees, 

 and which must be considered, in an eruptive sense, as passive. 



2. The porphyroid granite of the culminating points. And, 



3. The various granitic or feldspar rocks of Luchon, and the lower 

 parts of the valley. 



The last two have played an active or eruptive part in the formation 

 of the Pyrenees, and have produced veins, dislocations, unmistakeable 

 phenomena of metamorphism, &c. 



11 ■^'^^ P^'^^^^P^l facts in tlie above have been extracted from a paper lately 

 aaaressed to the Academy of Sciences at Paris, by L. F. Menabrea, entitled— 

 if/r i -^b^cs entre Modane et Bardonkke, which may be con- 



suited by those of our readers who are desirous of further technical details.— T. L. P. 



