p. JF. Stuart- Menteath — Salt Deposits of Dax, etc. 265 



VI.— The Salt Deposits op Dax and the Pyrenees. 

 By P. W. Stuart-Menteath, Assoc. E. S. "Mines. 



N the rail to Biarritz the roots of the Pyrenees first appear at 

 Dax, and are accompanied by those ophites and thermal 

 springs which are special features of the entire chain. Vast deposits 

 of salt, to whose first development I contributed, have added an 

 important industry to the resources of this ancient capital oi Aqiw 

 Tarbelliccs, where the exact harness depicted on Roman medals 

 is still characteristic of every cart. Beneath the existing ditch of 

 the Eoman fortifications rock-salt was accidentally discovered by 

 a boring for mineral water, and the salt is now worked at three 

 miles to the south-east, and is indicated by springs for a distance of 

 seven miles. The deposit is known to be about 100 feet in thickness, 

 but is of unknown depth beneath the existing borings. 



Along the entire outskirts of both sides of the Pyrenees similar 

 salt deposits abound, and they are often similarly accompanied by 

 igneous rocks. 



The salt formation of Dax is distinctly limited by the valley of the 

 Adour, which here ceases to wander among the sands of the plain, 

 and is suddenly and sharply diverted along a tectonic depression, 

 running towards the Pyrenees in a south-west direction. Precisely 

 parallel to this course, in the Cretaceous aud Tertiary rocks of the 

 Pyrenees, there runs, at a dozen miles to the north-west, the most 

 remarkable example known of a tectonic valley sunk beneath the 

 ocean. The Gouf de Capbreton, sinking with steep sides to over 

 3,000 feet beneath the even bottom of the Atlantic skirt, and 

 afi'ording evidence of igneous rocks in its surroundings and in the 

 irregularities of its floor, is a perfect analogue of the neighbouring 

 tectonic portion of the Adour. One is disposed to attribute the 

 salt deposits of the Pyrenees to an episode in the past history of 

 such valleys, whereby they were upraised, with salt lagoons in 

 their irregular hollows, and with rapid evaporation of the brine 

 by volcanoes such as accompany salt lakes of Eastern Africa. The 

 disposition of Pyrenean salt accords fairly with such a theory ; but 

 matters of engineering importance are not usually decided by any 

 royal road of first impressions, however plausible or fascinating. 



Still more important than the salt of Dax are those thermal springs 

 which, along ten miles of the tectonic valley of the Adour, form 

 a western limit to the salt by an emanation of over 5,000 tons daily of 

 mineral water at a temperature of 147° Fahrenheit. This water, by 

 impregnating the mud of the Adour, excites a growth of confervce, 

 -diatoms, and other organisms, that may develop to even half the 

 weight of the whole material. They transform the complex mass, 

 with production of nascent oxygen from the carbonic acid that 

 accompanies the abundant nitrogen which bubbles from the springs. 

 Such actions, as explained by Bischof, doubtless originate the powerful 

 therapeutic action of the Dax mud baths. One may attribute to such 

 mud the variegated marls which accompany the salt deposits and 



