p. W. Stuart-Menteath—Salt Deposits of Dax, etc. 269 



fossils in black asphalt, in great abundance beside the ruins of the 

 old workings at Bastennes. At Tercis, near Dax, the same horizon 

 of abundant Greensand fossils forms the lowest visible beds above 

 the variegated marls which contain ophite, salt, and gypsum. But 

 from beneath these variegated marls there outcrops, at Le Hour, 

 at a trifling distance to the south-west of the salt-mine, a thick band 

 of dolomitic limestone and breccias, alternating with beds of ash, 

 and pinched between extensive outcrops of ophite. This limestone 

 is absolutely identical with the Muschelkalk, which I have found 

 fossiliferous from St. Michel to Ascain, along many miles of the 

 nearest Pyrenees. It is strangely identical with the most typical 

 Muschelkalk of the Hartz, it presents both the peculiar dolomitic 

 breccias and the blue and green Aerinite which characterize Triassic 

 beds accompanying ophite at Camarasa in Catalonia, and it abounds 

 in moulds of Gastropods, etc., such as peculiarly characterize the 

 Muschelkalk of the Pyrenees. In a former paper I denied this 

 identity, because the most characteristic portions of the rock had 

 been largely removed by quarrying ; but having found them by 

 repeated later visits, I can recognize the identity in both character 

 and relative situation of this typical Muschelkalk. This rock was 

 found to be fossiliferous by Crouzet and De Freycinet, and its fossils 

 classed as Tertiary ; Eaulin and others classed it as Cretaceous ; 

 Jacquot compared it to the Muschelkalk in 1888 ; M. Seunes 

 declared its fossils to be of the infra-Lias in 1890. As both at Le Hour, 

 Ascain, and many other localities, it is beneath a considerable thick- 

 ness of Keuper marls, and as the fossils habitually resemble those of 

 the Muschelkalk, the last-mentioned determination is inadmissible 

 and misleading. The real infra-Lias, with ^stJieria minuta, fish 

 spines, and other remains, I have found at Elduayen, Villabona, etc., 

 and it is different in appearance, as well as closely connected with 

 the Lias, containing Gryphcea arciiata at Narvarte in the Bastan. 



At Dax we have consequently the Trias with salt, gypsum, etc., 

 rising in ridges and bosses from beneath all later rocks, and these 

 outcrops are occasioned by the presence of igneous intrusions that 

 have given rise to abundant thermal springs charged with salt. 

 Obviously this machinery can transfer the salt and gypsum to any 

 depressions or lagoons formed even to the present day, and we 

 have consequently a sheet of salt and gypsum laid down in the 

 extensive depression which borders the tectonic valley of the Adour. 

 This sheet appears to date from the latest vicissitudes of the district, 

 and certainlj' from later than the last upheaval of the Pyi-enees. 

 Wherever we find similar machinery of Triassic bosses below, 

 igneous intrusions breaking and dislocating that Trias, springs con- 

 veying its contents to the surface, and tectonic irregularities and 

 barriers occasioned by the movements of the Pyrenees, we may 

 naturally find salt deposits of any age later than the Trias. The 

 detection of their presence, and the estimation of their depth and 

 extent, is a problem special to each particular district, and depending 

 on the entire geological history which can be worked out on the spot. 



The vast sheets of gypsum and salt that extend from Olot to 



