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Show 208 MIOCENE PERIOD, [Ch. XV. compact fresh-water limestone, of slight thickness, which is pel'forated on the upper surface by marine shells, for the most part of extinct species. It is evident that the space mus~ have been alternately occupied by salt and fresh water. F1rst, a laO'oon may have been formed, in which the water may have be~ome fresh; then a barrier of sand, by which the sea was excluded fot· a time, may have been breached, whereby the salt water again obtained access. Eocene strata in the Bordeaux basin.-The relations of some of the mem hers of the tertiary series, in the basin of the Gironde, have of late afforded matter of controversy. A limestone, resembling the calcaire grossier of Paris, and from 100 to 200 feet in thickness, occurs at Pauliac and Blaye, and extends on the right bank of the Gironde, between Blaye and La Roche. It contains many species of fossils identical with those of the Paris basin. 'l~his fact was pointed out to me by M. Deshayes before I visited Blaye in 1830; but although I recognized the mineral characters of the rock to be very different from those of the Miocene formations in the immediate neighbourhood of Bordeaux, I had not time to verify its relative position. I inferred, however, the inferiority of the Blaye limestone to the Miocene strata, from the order in which each series presented itself as I receded from the chalk and passed to the central parts of the Bordeaux basin. Upon leaving the white chalk with flints, in travelling from Charente by Blaye to Bordeaux, I £rst found myself upon overlying red clay and sand (as at Mirambeau); I then came upon the tertiary limestone above alluded to, at Blaye; and lastly, on departing still farther from the chalk, reached the strata which at Bordeaux and Dax contain exclusively the Miocene shells. The occurrence both of Eocene and Miocene fossils in the same basin of the Gironde, had been cited by M. Boue as a fact which detracted from the value of zoological characters as a means of determining the chronological relations of tertiary Ch. XV.] INLAND CLIFF NEAR DAX. 209 gron.ps. But. ~n farther inquiry, the fact, on the contrary, has fumished additional gl'Ounds of confidence in these characters. . M. Ch. Desmoulins replied, in answer to M. Boue's objcctwns, that the assemblage of Eocene shells are never intermixed with those found in the ' moellon,' as he calls the sandy calcareous rock of the environs of Bordeaux and Dax; and M. Dufrenoy farther stated, that the hills of limestone which border the right bank of the Gil'Onde, from Marmande as far as BJaye, present several sections wherein the Parisian (or Eocene) limestone is seen to be separated from the sheliy strata called t faluns,' or r moellon,' by a fresh-water formation of considerable thickness. It appears, therefore, that as the marine faluns of Touraine rest on a fresh-water formation, which overlies the marine calcaire grossier of Paris, so the marine Miocene strata of Bordeaux are separated from those of Blaye by a freshwater deposit*. The following diagram, therefore, will express the order of position of the groups above alluded to. No. 62, ~ c--_L r d d ~ ~~s----------~--- a, Red clay and sand. b, Limestone like calcaire grassier, sometimes alternating with green marl and containing Eocene shellY, c, Fresh-water formation, same as that of the department of Lot and Garonne, d, Tertiary strata of the Landes, with Miocene fossils, Inland cliff near Dax.-A few miles west from Dax and at ~he distance. of ~bout. twe:ve miles from the sea, a ste~p bank Is seen runmng In a directiOn nearly north-east and south-west or parall~l t~ the contiguous coast. This steep declivity, 0 ; b~ae, whiCh Is about 50 feet in height, conducts us from the lugher platform of the Landes to a lower plain which extends to the .sea. The outline of the ground might sugO'est to every geologist the opinion, that the bank in questio~ was once * Bulletin de Ia Soc, Geol. de France tome ii p 440 Vot,lii, ' ' ' ' p |