558 



THE GEOLOGIST. 



Additional Note on the Geology of Biarritz. — Biarritz is built 

 chiefly on a soft sand, passing sometimes into a clay apparently of fresh water 

 origin. These bads are very loosely composed and appear to have been much 

 distured by subsidence and slight landslips, they rest unconformably on a sand- 

 stone-rock abounding in the numulitic fossils, also in echini and shells. Going 

 south from Biarritz, that is towards the Spanish side, you find the sandstone- 

 rock passing into a blue clay. The sand rock overlies the clay ; at their junc- 

 tion there is much disturbance, but it is clear that the blue clay underlies the 

 sandstone. This is well seen between Vieux Port and the Basque sands. 

 Trom the point where the blue clay cliff begins it is very regular in its struc- 

 ture. It continues for at least a mile, dipping uniformly throughout that 

 distance to the north-west at an angle of 45 degs. ; the lines of stratification 

 are well defined by bands of stone (a sort of clay stone) lighter in colour and 

 harder in texture than the mass of the cliff, the day of which is soft and much 

 worn into furrows by the weather, and by the little streams which flow down 

 it. Here and there the cliff is capped with beds of sand lying horizon- 

 tally on regular strata of yellow, white, and pink colour, much resembling 

 the tertiary sands of Alum Bay in the Isle of Wight. 



At the end of these clay cliffs which gradually sink down to the shore, you 

 find first recent sand-hills, and a little further beds of sand like those which cap 

 the cliff, lying horizontally. Where first they appear low down thus on the 

 shore, they are from fifteen to twenty feet high. The uppermost beds are of 

 yellow and brownish sand mixed with pebbles ; beneath these is a band of 

 orange-coloured clay about three inches thick, very clearly defined, and imme- 

 diately below it a dark clay passing downwards into dark brown, and sometimes 

 almost black vegetable matter. This, when dry, splits into thin layers like 

 card-board, it is full of roots and of stems of fir trees ; I also found impressions 

 of seeds and fir cones, besides masses of leaves of water plants. This bed 

 varies from a few inches to several feet in thickness. Further on a dark iron- 

 grey sand appeared beneath this bed, but this was only at one point. Going 

 still south the cliff rises again (after an interval of about half a mile) and 

 attains an average height of about forty feet. Here it is composed of the hor- 

 izontal sands, only with the band of orange clay and the fresh water vegetable 

 bed beneath as the base of the cliff. This continues for about a quarter of a 

 mile, and then the blue clay of the cliff near Biarritz reappears under the 

 horizontal sands, dipping as before at a sharp angle, but not so uniformly. At 

 one point its beds are thrown up quite edgeways, and the numulitic sandstone- 

 rock appears intruding beneath it, through the sea-shore sand. The clay 

 rests against the sandstone as if the latter had been forced up against it. Here 

 there is abundant evidence of great disturbance. 



Not many yards beyond where the sandstone first appears, it appears again so 

 different in texture, that it could hardly be recognised as the same rock, were 

 it not that it is rich in numulite like the softer rock. In the second instance 

 the soft, yellowish sandstone has been changed into a hard, white, and shining 

 rock, and where the clay rests against it there is a good deal of crystalization, 

 and the clay has been clianged into a rich, pinky brown. All the clay here is 

 much more disposed to be shaly than it is near Biarritz, and is harder in con- 

 sequence. 



Nearer the water at this point, especially at low tide, are several beds 

 quite perpendicular, of which the edges only obtrude ; they appear of a hard 

 and beautifully coloured rock, quite without trace of fossils. Near it I ob- 

 served some masses of a very dark green amorphous rock ; this is the rock 

 called by Mons. Guidres (page 44 of his little work) Ophite. — Yours, &c, 

 A. D. Acworth. 



Errata in Sir R, I, Murchison's Address to the Geological Sec- 



