Contents.
1. The unfossiliferous sands of the Red Crag.
2. The unconformity between the Lower and Middle Glacial beds; and the interglacial valley-excavation.
3. The consideration of the mode in which the Middle Glacial series was accumulated, and of the way in which the sequence of the beds posterior to the Lower Glacial series is to be traced.
In writing the Introduction to the 'Supplement to the Crag Mollusca,' issued by the Palæontographical Society (for 1871), our object was, with the help of the Map and Sections accompanying it, to give only such a compressed or synoptical account of the researches on which we had for several years been engaged as would enable geologists to perceive the results at which we had arrived respecting the succession of the beds posterior to the Crag in the East of England. We considered that as the officers of the Geological Survey had commenced the examination of the district, and would eventually publish a detailed account of the sections and other evidence bearing upon that succession, we should have only encumbered scientific publications by bringing forward in greater detail the physical evidence which we had collected, and which had led us to the conclusions of which we thus gave a representation, the evidence of organic remains which we had collected being given by the author of the 'Crag Mollusca' in the tabular lists which accompany the Supplement to that work.
There are, however, some subjects referred to in that "Introduction" upon which we have made subsequent observations that we desire to bring forward; and there is one in particular upon which we touched only slightly, the break between the Lower and Middle Glacial deposits, which from its geological importance it is desirable should be shown in some detail, in order that while the officers of the Survey are engaged upon the district it may receive from them the scrutiny which it merits.
1. The Unfossiliferous Sands of the Red Crag.
The first subject on which we have to remark is that of the unfossiliferous sands overlying the Red Crag. These sands have long been a subject of perplexity to one of us ; and in reference to them we observed, in the before-mentioned "Introduction" (p. viii), that "we did not think they represented the Chillesford sands, as they neither contained the Chillesford shell-bed, nor, though some-
