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. 150.

Voluta Lamberti, young individ.

The occurrence of a species of Lingula at Sutton is worthy of remark, as these Brachiopoda seem now confined to more equatorial latitudes, and the same may be said still more decidedly of a species of Pyrula, allied to P. reticulata. Whether, therefore, we may incline to the belief that the mean annual temperature was higher or lower than now, we may at least infer that the climate and geographical conditions were by no means the same at the period of the Suffolk crag as those now prevailing in the same region.

Of the echinoderms of the coralline crag about eleven species are known, but some of these are in too fragmentary a condition to admit of exact comparison. Of six which are the most perfect, Prof. E. Forbes has been able to identify three with recent species: one of which, of the genus Echinus, is British; a second, Echinocyamus, British and Mediterranean; and a third, Echinus monilis, a Mediterranean species, also found fossil in the faluns of Touraine.

One of the most interesting conclusions deduced from a careful comparison of the shells of these British Older Pliocene strata and those now inhabiting our seas, has been pointed out by Prof. E. Forbes. It appears that, during the glacial period, a period intermediate, as we have seen, between that of the crag and our own times, many shells, previously established in the temperate zone, retreated southwards to avoid an uncongenial climate. The Professor has given a list of fifty shells which inhabited the British seas while the coralline and red crag were forming, and which are wanting in the Pleistocene or glacial deposits. They must, therefore, after their migration to the south, have made their way northwards again. In corroboration of these views, it is stated that all these fifty species occur fossil in the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian Archipelago, where they may have enjoyed, during the era of floating icebergs, a climate resembling that now prevailing in higher European latitudes.[166-A]

In the red crag at Felixstow, in Suffolk, Professor Henslow has found the ear-bones of no less than four species of cetacea, which, according to Mr. Owen, are the remains of true whales of the family Balænidæ. Mr. Wood is of opinion that these cetacea may be of the age of the red crag, or if not that they may be derived from the destruction of beds of coralline crag. I agree with him that the supposition of their having been washed out of the London clay, in which no Balænidæ have yet been met with, is improbable.

Strata containing fossil shells, like those of the Suffolk crag, above described, have been found at Antwerp, and on the banks of the Scheldt below that city. In 1840 I observed a small patch of them near Valognes, in Normandy; and there is also a deposit containing similar fossils at St. George Bohon, and several places a few leagues to the S. of Carentan, in Normandy; but they have never been traced farther southwards.

Subapennine strata.—The Apennines, it is well known, are composed chiefly of secondary rocks, forming a chain which branches off from the Ligurian Alps and passes down the middle of the Italian peninsula. At the foot of these mountains, on the side both of the Adriatic and the Mediterranean, are found a series of tertiary strata, which form, for the most part, a line of low hills occupying the space between the older chain and the sea. Brocchi, as we have seen ([p. 105.]), was the first Italian geologist who described this newer group in detail, giving it the name of the Subapennines; and he classed all the tertiary strata of Italy, from Piedmont to Calabria, as parts of the same system. Certain mineral characters, he observed, were common to the whole; for the strata consist generally of light brown or blue marl, covered by yellow calcareous sand and gravel. There are also, he added, some species of fossil shells which are found in these deposits throughout the whole of Italy.

We have now, however, satisfactory evidence that the Subapennine beds of Brocchi belong, at least, to three periods. To the Miocene we can refer a portion of the strata of Piedmont, those of the hill of the Superga, for example; to the Older Pliocene, part of the strata of northern Italy, of Tuscany, and of Rome; while the tufaceous formations of Naples, of Ischia, and the calcareous strata of Otranto, are referable to the Newer Pliocene, and in great part to the Post-Pliocene period.