Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/497

 which, they spread, but, in spreading, retained at the same time evidences of the grouping they had when living.

After the deposit of the lower division of the Red Crag a subsidence of some 10 or 12 feet took place, and a new line of cliff, on a higher level, was formed at Sutton. Even the rise of the tides might almost be determined ; for we know that the Pholas crispata, the borings of which are spread over the upper shore and top of the lower cliff, lives only between water-marks. The perforated space occupies a zone about 5 or 6 feet deep (see fig. 21).

The increased prevalence of oblique lamination in the upper part of the lower Red Crag leads to the supposition that the sea was gradually silting up. To this, however, succeeded a movement of subsidence causing the erosion, by fresh submarine currents, of the surface of the lower division, and, as the subsidence gradually continued, to the accumulation of the upper division of the Red Crag.

From the circumstance of the lower part of these upper sands being formed of the debris of the previously deposited lower Red Crag, the mineral character is very similar ; but in the ascending- series, formed as the sea-bed gradually became more depressed, the sands are finer, they lose their red colour, and they pass upwards into fine micaceous sands and grey clays, in which the Testacea are constantly found on the spot where they lived.

These sands and laminated clays (the Chillesford series) covered up and levelled the whole of the various beds beneath them, spreading over them all through Suffolk and Norfolk.

The difference of age which the distinctive characters of the Red and Coralline Crags might suggest does not hold when each is viewed separately in relation to the fauna of the present period. There must have been other causes than mere lapse of time and descent to account for this variation in their respective faunas ; and this probably was owing both to the variation of the level of the bed of the sea, to which we have directed attention, and by which many of the deeper-water Mollusca must have been destroyed within our area, and also to the influx of more northern or north-easterly currents, and the increased cold, indicated by the physical phenomena, bringing in a more northern fauna. It is a question of temperature rather than of time.

The absence of essential difference of age between the two crags is also apparent in their correlation with the two upper Crags of Antwerp, with both of which they show a close relationship — the lower one of the two being probably of the age of the Coralline, and the upper one having a near analogy with the Red Crag.

Antwerp beds, number of species.

Species common to the Antwerp beds and the Red Crag.

Proportion of Red-Crag species.

Scaldisien Sables jaunes 197 138 74 per cent.

Sables gris 187 122 65 „

Diestien Sables noirs 228 61 26 „

Common to the Sables jaunes and gris 139, or 75 per cent.

We will, however, consider more fully the relations of the Belgian