The Antiquity of Man

by Sir Charles Lyell

TABLE 12/3. PROPORTION OF RECENT TO EXTINCT SPECIES.

COLUMN 1: NAME.
COLUMN 2: NUMBER OF RECENT.
COLUMN 3: NUMBER OF EXTINCT.
COLUMN 4: PERCENTAGE OF RECENT.
Norwich Crag: 69: 12: 85%.
Red Crag: 130: 95: 57%.
Coralline Crag: 168: 159: 51%.
TABLE 12/4. RECENT SPECIES NOT LIVING NOW IN BRITISH SEAS.
COLUMN 1: NAME.
COLUMN 2: NUMBER OF NORTHERN.
COLUMN 3: NUMBER OF SOUTHERN.
Norwich Crag: 12: 0.
Red Crag: 8: 16.
Coralline Crag: 2: 27.

In the above list I have not included the shells of the glacial beds of the Clyde and of several other British deposits of newer origin than the Norwich Crag, in which nearly all—perhaps all—the species are Recent. The land and freshwater shells, thirty-two in number, have also been purposely omitted, as well as three species of London Clay shells, suspected by Mr. Wood himself to be spurious.

By far the greater number of the living marine species included in these tables are still inhabitants of the British seas; but even these differ considerably in their relative abundance, some of the commonest of the Crag shells being now extremely scarce; as, for example, Buccinopsis Dalei; and others, rarely met with in a fossil state, being now very common, as Murex erinaceus and Cardium echinatum.

The last table throws light on a marked alteration in the climate of the three successive periods. It will be seen that in the Coralline Crag there are twenty-seven southern shells, including twenty-six Mediterranean, and one West Indian species (Erato Maugeriae). Of these only thirteen occur in the Red Crag, associated with three new southern species, while the whole of them disappear from the Norwich beds. On the other hand, the Coralline Crag contains only two shells closely related to arctic forms of the genera Admete and Limopsis. The Red Crag contains, as stated in the table, eight northern species, all of which recur in the Norwich Crag, with the addition of four others, also inhabitants of the arctic regions; so that there is good evidence of a continual refrigeration of climate during the Pliocene period in Britain. The presence of these northern shells cannot be explained away by supposing that they were inhabitants of the deep parts of the sea; for some of them, such as Tellina calcarea and Astarte borealis, occur plentifully, and sometimes, with the valves united by their ligament, in company with other littoral shells, such as Mya arenaria and Littorina rudis, and evidently not thrown up from deep water. Yet the northern character of the Norwich Crag is not fully shown by simply saying that it contains twelve northern species. It is the predominance of certain genera and species, such as Tellina calcarea, Astarte borealis, Scalaria groenlandica, and Fusus carinatus, which satisfies the mind of a conchologist as to the arctic character of the Norwich Crag. In like manner, it is the presence of such genera as Pyrula, Columbella, Terebra, Cassidaria, Pholadomya, Lingula, Discina, and others which give a southern aspect to the Coralline Crag shells.

The cold, which had gone on increasing from the time of the Coralline to that of the Norwich Crag, continued, though not perhaps without some oscillations of temperature, to become more and more severe after the accumulation of the Norwich Crag, until it reached its maximum in what has been called the glacial epoch. The marine fauna of this last period contains, both in Ireland and Scotland, Recent species of mollusca now living in Greenland and other seas far north of the areas where we find their remains in a fossil state.

The refrigeration of climate from the time of the older to that of the newer Pliocene strata is not now announced for the first time, as it was inferred from a study of the Crag shells in 1846 by the late Edward Forbes.*

(* "Memoirs of the Geological Survey" London 1846 page 391.)

The most southern point to which the marine beds of the Norwich Crag have yet been traced is at Chillesford, near Woodbridge, in Suffolk, about 80 miles north-east of London, where, as Messrs. Prestwich and Searles Wood have pointed out,* they exhibit decided marks of having been deposited in a sea of a much lower temperature than that now prevailing in the same latitude. (* "Quarterly Journal of the Geological Society" volume 5 1849 page 345.) Out of twenty-three shells obtained in that locality from argillaceous strata 20 feet thick, two only, namely, Nucula Cobboldiae and Tellina obliqua, are extinct, and not a few of the other species, such as Leda lanceolata, Cardium groenlandicum, Lucina borealis, Cyprina islandica, Panopaea norvegica, and Mya truncata, betray a northern, and some of them an arctic character.

These Chillesford beds are supposed to be somewhat more modern than any of the purely marine strata of the Norwich Crag exhibited by the sections of the Norfolk cliffs north-west of Cromer, which I am about to describe. Yet they probably preceded in date the "Forest Bed" and fluvio-marine deposits of those same cliffs. They are, therefore, of no small importance in reference to the chronology of the glacial period, since they afford evidence of an assemblage of fossil shells with a proportion of between eight and nine in a hundred of extinct species occurring so far south as latitude 53 degrees north, and indicating so cold a climate as to imply that the glacial period commenced before the close of the Pliocene era.