PEESTWICH — CEAG-BEDS OF STJPFOIiE; AKB NOEFOLK. 135. 



a transport from adjacent Eocene land, while tlie 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 conti-; 

 nuing, beds " c " and " d" Avere deposited in comparatively deep and 

 tranquil water. These beds are succeeded by the sands " e," abound- 

 ing 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 ele- 

 vation, and the setting in of stronger cui'rents 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 "/" and " e " in banks over portions of the 

 sea-bed. Under such conditions the upper division, "^r," 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 gradu- 

 ally 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 subsi- 

 dence, 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 intro- 

 duction 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, Glyclmeris siliqua, Necera jugosa, Tellina 

 calcaria, Buccinopsis Dalei, Geritliium c/ranosum, Emarginula crassa, 

 Piliscus commodas, Puncturella Noachina, Tectwa fidva, and Tricho- 

 tropis borealis, amongst the Bryozoa the Retepora Beaniana, and 

 amongst the Foraminifera the Lagena globosa and L. ornata — all 



