Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/230

116 paper forward until I was in possession of facts which might afford sufficient grounds for the conclusions I now venture to submit to the Society. The whole question is intimately connected; yet, as the geological series is divisible into distinct stages, I will take each of these separately, commencing with the lowest.

Coralline Crag.—The area of the Coralline Crag has not been extended since Mr. Charlesworth first drew attention to it, giving the neighbourhood of Orford as its centre, with outliers at Aldborough, Sutton, Ramsholt, and Tattingstone. The boundaries only are better known. The extent of superficial area exposed is about eight square miles. Originally the Coralline Crag may have extended uninterruptedly from Aldborough to Tattingstone; but, with the exception of the low range of hills extending from Gedgrave northward to Orford, Sudbourne, and Iken, and the small outlying masses of Aldborough, Sutton, and Tattingstone, it has everywhere been removed by denudation. Not only did this denudation remove the Coralline Crag, but it has also removed a portion of the underlying London Clay; so that the base of the Red Crag is in places lower than that of the older Coralline Crag, round and over which it wraps and passes transgressively.

The surface of the London Clay under the Coralline Crag is also uneven. In the Bullock-yard pit, on Mr. Colchester's farm at Sutton, it is found under 4 feet of Red and 2 of Coralline Crag, and 20 feet above high tide of the river Deben; but an eighth of a mile to the west the London Clay is 12 feet lower, and a lower zone of the Coralline Crag comes in. This and other circumstances lead me to believe that at the noted old pit at Ramsholt the Coralline Crag, which there lies on the London Clay, does not belong to the lowest zone, but to one some 10 to 15 feet higher.

The well-known outlier of Sutton supplies us with a typical exhibition of the Coralline Crag, the several pits which have from time to time been opened there giving us the best clue to its structure and dimensions, whilst at the same time the extent of denudation by the Red Crag, and the varying levels of the sea during the deposition of these latter beds, are well shown (see Plan and Sections, Pl. VI.).

It is generally known that the Coralline Crag consists of two divisions—an upper one, formed chiefly of the remains of Bryozoa, and a lower one of light-coloured sands, with a profusion of shells. The more exact dimensions and subdivisions of these beds at Sutton, Orford, Sudbourne, and Gedgrave I now purpose to give.

The discovery of the so-called Coprolites in the Red Crag by the late Professor Henslow, in 1848, led to a great extension of crag-pits. With one exception they were all in the Red Crag. The only one in the Coralline Crag was opened by Mr. Colchester, on the south side of Sutton-farm Hill (, Pl. VI.). Unfortunately the pit did not prove remunerative, and a year or two later it was filled up, and the ground levelled; so that it was only seen by myself and Mr. Ray Lankester, as mentioned by him in a paper read before this Society. The section was of much interest, as it exposed beds which belong, I believe, to the lowest zone of the Coralline Crag, and showed