Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/224

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Mr. Webster on the Strata lying over the Chalk.

the Hordwell cliffs. It is very probable therefore that traces of this formation may be found there, if this spot were well examined for the purpose.[1]

Land and river shells have been repeatedly discovered in various parts of England; and often at some depth under beds of sand and gravel. They are then often accompanied by the bones of land animals as those of the elephant, hippopotamus, &c. and may be referred to a very ancient period probably connected with some of these formations. None of them however had as yet been discovered imbedded in a stratum of rock. When they have been found under peat bogs they have been most probably produced in some of the later states of the earth.

It is in this formation, in the Paris basin, that the gypsum beds are placed. Three series of gypseous strata are described: the lowermost consists of thin beds of gypsum, often selenitous, of solid calcareous marls, and foliated argillaceous marls; and they observe, that these are sometimes deposited on the marine shelly calcareous sand, and then they contain marine shells. In these are also beds of white marl, containing a great quantity of freshwater shells of the genera limneus and planorbis. These lower beds, with the white marl, constitute the oldest freshwater formation; and, according to the observations of M. M. Cuvier and Brongniart, appear to have been formed during the passage or change of the Paris basin from the state of a marine bay to that of a freshwater lake. The second mass of beds differs from the above mentioned

  1. Since the reading of this paper I have been favoured by the Rev. William Buckland, Prof. of Mineralogy, Oxford, with specimens of freshwater shells which he has collected at Hordwell cliff. They consist of the lymneus, planorbis, cyclostoma, and a bivalve resembling a freshwater mytilus. From the state of the fossils, and the nature of the stratum in which they were imbedded, they would appear to belong to the lower freshwater formation.