INTRODUCTION. ill 
end of Section A), another at Pettistree Hall, Sutton (see also Section A), another at 
Ramsholt Cliff (on the Deben River, near Sutton), and the main mass which occupies 
the parishes of Orford, Sudbourn, Iken, and Aldborough. To these may be added a 
trace (not shown in the map) at Trimley, where it was observed in the digging of a 
ditch by the late Mr. Acton. 
The Cor. Crag has long been known to consist of two main portions, and a third 
subordinate bed. The first and lowest of these, (3’ of Section XXII) consists of a series 
of calcareous sands, in some places more or less marly, which are rich in Molluscan remains. 
The second, 8”, consists of a solid bed formed of Molluscan remains, agglutimated with the 
fronds and fragments of various species of Polyzoa into a rock, so hard as to have been 
formerly quarried for building. The third and uppermost, 3’, is a thin subsidiary bed, 
consisting of a few feet of the abraded material of the rock, reconstructed evidently in 
very shoal water, probably, indeed, between tide marks, as it is very obliquely bedded. 
From the outliers at Tattingstone and Ramsholt this rock bed is absent, but over 
the Sutton outlier a small cap of it remains. Over the main mass, however, it 
spreads continuously, and either from a slight northerly dip of the whole formation, or 
else from a displacement of the underlying shelly sands, this rock bed descends to the 
sea level at the northern extremity of the area. The thickness of the formation has been 
estimated at between eighty and ninety feet, but this seems to us to be much in excess 
of the fact. The place to test the true thickness of the formation is clearly that where it 
is in the greatest state of preservation. This is the neighbourhood of Sudbourn, at the 
southern extremity of the main mass. ‘The London clay, upon which it rests, comes out 
along the Butley Creek Marshes, and the shelly beds in their full force appear along the 
slope which frmges those marshes on the eastern side of the creek, where they are 
exposed in several pits, known as the Gedgrave, the Gomer, the Broom, and the Hall 
pits, from which the Mollusca of that neighbourhood are obtained. Higher up this 
slope comes the rock bed, which forms the upper part of the low hills, and this is 
exposed in numerous pits, that stretch away from those eminences to the northern 
extremity of the formation. The outcrop of the shelly beds (3’) along this slope is shown 
in Section XXII. Now, it is clear, as it seems to us, that, the whole formation being 
thus present in the complete state, we have only to take the elevation of the highest 
points attained by the Cor. rock bed, 8”, and its overlying reconstructed bed, 
3’, m these parts, above the elevation at which the London clay thus crops out at its 
base, and the difference, after making allowance for any slight dip there may be, will be 
the thickness of the entire mass. Hstimated in this way, it will be difficult to make out 
the thickness of the Cor. Crag as exceeding sixty feet. 
Mr. Prestwich has attempted to divide the shelly sands, 3’, into constant and de- 
terminable horizons, which he thinks might by investigation be identified by special 
groups of fossils. We doubt the constancy or determinability of such horizons ; and, so far 
from their being characterised by special groups of fossils, the author of the ‘ Crag Mollusca’ 
