A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Fig. 188.

Melania inquinata, Des. Nat. size.

Syn. Cerithium melanoides, Min. Con.

With us as in France, clay of this formation is used in some places, as near Poole in Dorsetshire, for pottery; and hence the name of plastic clay was adopted for the group by Mr. T. Webster. Lignite also is associated with it in some spots, as in the Paris basin.

Before the minds of geologists had become familiar with the theory of the gradual sinking of the land, and its conversion into sea at different periods, and the consequent change from shallow to deep water, the freshwater and littoral character of this inferior group appeared strange and anomalous. After passing through many hundred feet of London clay, proved by its fossils to have been deposited in salt water of considerable depth, we arrive at beds of fluviatile origin. Thick masses, also, of shingle indicate the proximity of land, where the flints of the chalk were rolled into sand and pebbles, and spread continuously over wide spaces, as in the Isle of Wight, in the south of Hampshire, and near London, always appearing at the bottom of the Eocene series. It may be asked why they did not constitute simply a narrow littoral zone, such as we might look for in strata formed at a moderate distance from the shore. In answer to this inquiry, the student must be reminded, that wherever a gently-sloping land is gradually sinking and becoming submerged, shingle may be heaped up successively over a wide area, although marine currents have no power of dispersing it simultaneously over a large space. In such cases it is not the shingle which recedes from the coast, but the coast which recedes from the shingle, which is formed one mass after another as often as successive portions of the land are converted into sea and others into a sea beach.

The London area appears to have been upraised before that of Hampshire, so that it never became the receptacle of the Barton clays, nor of the overlying fluvio-marine and freshwater beds of Hordwell and the north part of the Isle of Wight. On the other hand, the Hampshire Eocene area seems to have emerged before that of Paris, so that no marine beds of the Upper Eocene era were ever thrown down in Hampshire.

Nummulitic formation of the Alps and Pyrenees.—It has long been matter of controversy, whether the nummulitic rocks of the Alps and Pyrenees should be regarded as Eocene or Cretaceous; but the number of geologists of high authority who regard this important group as belonging to the lowest tertiary system of Europe has for many years been steadily increasing. The late M. Alex. Brongniart first declared the specific identity of many of the shells of this formation with those of the marine strata near Paris, although he obtained them from the summit of the Diablerets, one of the loftiest of the Swiss Alps, which rises more than 10,000 feet above the level of the sea.

Deposits of the same age, found on the flanks of the Pyrenees, contain also a great number of shells common to the Paris and London areas, and three or four species only which are common to the cretaceous formation.