AND GEOLOGICAL RELATIONS OF CERTAIN HOT SPRINGS. 
589 
IV. St. Sauveur. 
A. Geological Position . — The site of St. Sauveur is justly celebrated for its beauty. 
Situated upon a shelf in a steep acclivity, the little town commands views of the 
Valley of Bareges or Lavedan, the entrance of the Valley of Bastan, and the Pas des 
Echelles, leading to Gavarnie. The rock upon which it is built is a blue slaty lime- 
stone, but exhibiting some variety of structure : this limestone, I presume, belongs to 
the transition series, but it has some very interesting relations to other rocks. We 
are here upon the margin of those hornblende slates to which allusion has already 
been made, which Charpentier calls primitive trap, and to which he has also given 
the less exceptionable name of the System of Bareges. It seems to me that there is 
a remarkable connexion between the geological positions of Cauteretz, St. Sauveur, 
and Bareges. I cannot doubt that these slates are altered rocks ; and it is natural to 
attribute the alteration to the near vicinity of granite in all these cases. Above 
St. Sauveur, these slates are separated by a very small interval from the granite 
forming the Valley of Lutour, and the bold summits which divide that valley from 
the Pas des Echelles. This is the same mass which extends to Cauteretz. Again, 
near Bareges, we see these slates in contact with sienites and granites near the Lac 
d’Escoubous. At that point, too, we find felspar beds or veins intermixed wfith the 
slates, and these even extend to form mountain masses, as, for instance, the barrier of 
the lake just named. I have little doubt that these veins communicate directly with 
the granitic chain ; and we have near the same spot (in the Pic d’Escoubous) exam- 
ples of granitic veins similar to those of Cornwall. I cannot doubt that the slates of 
the Pic d’Ereslids, near the above, owe their extreme hardness, their peculiarities of 
mineral composition, and vertical position, to the action of the granite and its tribu- 
tary veins. Now in the magnificent section afforded by the ravine immediately above 
St. Sauveur, called the Pas des Echelles, we recognise a similar series of rocks, and 
these too have frequent veins or beds of felspar intermixed : the strata are almost 
vertical, and their direction is parallel to those of the Pic d’Ereslids, stretching from 
N.W. to S.E. by the compass. Now the limestone from which the springs of St. Sau- 
veur issue, seems nearly to coincide with that which occurs on the opposite side of 
the valley, at a spot well known to mineralogists, named the Ravin de Rioumaou. 
The limestone bed which there appears is worked for useful purposes. It coincides 
precisely with the commencement of the trap slates of Charpentier ; itself contains 
numerous minerals, such as prehnite (koupholite) and stilbite, usually associated 
with trap. Besides this it is distinctly rendered crystalline by the contact of felspar 
rock, and at the same time metalliferous, containing iron pyrites abundantly, and 
also, according to Charpentier, arsenical nickel and gray cobalt ; then follow the 
very remarkable series of hornblende and other slates, which form the walls of the 
chasm of the Pas des Echelles, as far as the Pont de Sias, above which limestone re- 
appears ; and that this chasm owes its origin to convulsion, and not to erosion, there 
