Ch.XV.] STRATA NEAR DAX. 207 
are often covered by a thin pellicle only of tertiary strata, 
which rests horizontally on the chalk and does not always 
conceal it. 
Adour R. Luy R. Puy Arzet. No. 51. 
Tertiary strata overlying chalk in the environs of Dax. 
a, Siliceous sand without shells. c, Sand and marl with shells. 
b, Gravel. d, Blue marl with shells. 
E, Chalk and volcanic tuff. 
In the valleys of the Adour and Luy, sections of all the 
members of the tertiary series are laid open, but the lowest blue 
marl, which is sometimes 200 feet thick, is not often pene- 
trated. On the banks of the Luy, however, to the south of 
Dax, the subjacent white chalk is exposed in inclined and 
vertical strata. In the hill called Puy Arzet the chalk, charac- 
terized by its peculiar fossils, is accompanied by beds of volcanic 
tuff, which are conformable to it, and which may be considered 
as the product of submarine eruptions which took place in 
the sea wherein the chalk was formed. 
About a mile west of Orthes, in the Bas Pyrenees, the blue 
marl is seen to extend to the borders of the tertiary formation, 
and rises to the height probably of six or seven hundred feet. 
In that locality many of the marine Miocene shells preserve 
their original colours. This marl is covered by a considerable 
thickness of ferruginous gravel, which seems to increase in 
volume near the borders of the tertiary basin on the side of the 
Pyrenees. 
In an opposite direction, to the north of Dax, the shelly 
sands often pass into calcareous sandstone, in which there are 
merely the casts of shells as at Carcares, and into a shelly brec- 
cia resembling some rocks of recent origin which I have re- 
ceived from the coral reefs of the Bermudas. 
Fresh- water limestone at Saucats. — Associated with the Mio- 
cene strata near Bordeaux, at a place called Saucats, is a 
