Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/249

Rh a transport from adjacent Eocene land, while the occurrence of flints shows the proximity of some Chalk shore.

We have therefore in this basement-bed evidence of the sea gaining on the land, and of the drifting of ice-carried boulders. As the land subsided the coarser materials of the basement-bed were covered up by a bed of comminuted shells. This subsidence continuing, beds "c" and "d" were deposited in comparatively deep and tranquil water. These beds are succeeded by the sands "e," abounding in Bryozoa, with small Echini, and a number of small bivalves, indicating apparently the greatest depth of sea (possibly of from 500 to 1000 feet) attained during the Coralline-Crag period. A change then took place, and a bed of comminuted shells, with occasional oblique lamination, was spread over this deep-sea bed, indicating possibly a shallowing of the sea by a reverse movement of elevation, and the setting in of stronger currents with intervals of quiet deposition. Farther elevation, exposing the sea-bed to the action of tides and currents, led to considerable wear and denudation of the lower beds and to the heaping up of the remains of Bryozoa and of Mollusca of beds "f" and "e" in banks over portions of the sea-bed. Under such conditions the upper division, "g" of the Coralline Crag seems to have been generally formed; at a few places only do some of the beds seem to have been formed tranquilly. I know of no more illustrative geological instance of the wearing action of sea-currents than the reconstruction of the banks of comminuted Mollusca and Bryozoa which constitute this upper division of the Coralline Crag.

Bed "h" shows, in the finer state of comminution of the shells and Bryozoa, that the water probably continued to get shallower; and finally a continuance of the same movement of elevation gradually raised the Coralline Crag above the sea, and exposed it to the denuding action which has removed so large a portion of it. Then, or during the Red-Crag period immediately following, the Coralline Crag was broken up into detached islands and reefs, amongst which the Red Crag was deposited during a period of slow and small subsidence, as I hope to show in the next part of this paper.

The more southern forms of Mollusca which had migrated thus far north during the Falunian period and that of the "Sables Noirs" of Belgium are replaced in the Coralline Crag by an assemblage of forms partly of southern range with others of a northern type. Either a general lowering of the temperature, or else the setting in over this area of fresh currents from the north (more probably the latter, as the Coralline-Crag fauna is not a littoral one), owing to the continued subsidence of land in that direction, led to the introduction of northern forms of life and the gradual extinction of more southern forms. Amongst the Mollusca we thus see several northern forms, as Astarte undata, Glycimeris siliqua, Neoera jugosa, Tellina calcaria, Buccinopsis Dalei, Cerithium granosum, Emarginula crassa, Piliscus commodus, Puncturella Noachina, Tectura fulva, and Trichotropis borealis, amongst the Bryozoa the Retepora Beaniana, and amongst the Foraminifera the Lagena globosa and L. ornata—all