ampton Sand, those which define the separation of the Middle from the Upper Division everywhere obtain ; for the presence of the sand-bed (generally with its characteristic vertical plant-markings, and designated by Mr. Judd the " Lower Estuarine Series ") is observable wherever the Northampton Sand and higher beds are found in the same section.
It is a question, therefore, whether it would not be wise to abandon the hard line of separation between these Middle and Lower Divisions, and to class them together as the " Ferruginous " or marine beds of the Northampton Sand, retaining the distinction of the Upper Division of that formation under Mr. Judd's name of the " Lower Estuarine Series."
Exception has been made to the use of Mr. Judd's terms " Upper Estuarine " and " Lower Estuarine." I shall not enter upon this question : Mr. Judd is well able to maintain his own position. But, as these terms have been adopted in the maps of the Geological Survey, and will doubtless also be used in Mr. Judd's forthcoming Memoir, I have deemed it well, for the avoidance of confusion, to retain them in this treatise. For the same reason, I have adopted Mr. Judd's term of the " Great Oolite Clay " for the clay overlying the Great Oolite Limestone.
It is a fact worthy of notice that the two series of beds, the " Upper Estuarine " and the " Lower Estuarine," — so widely separated in time and character, the one belonging to the period of the Great Oolite, and the other to that of the Inferior Oolite, — occur together in vertical juxtaposition in the neighbourhood of Northampton, throughout a large district including parts of both divisions of the county, and in Oxfordshire. In the latter county, the Upper Estuarine is traceable through to the Stonesfield Slate-bed ; and the difficulty of separating the two Estuarines in Oxfordshire led to the conclusion arrived at, and published by the Geological Survey in 1860, that the Northampton Sand (few of its fossils being then known) was equivalent to the Stonesfield Slate.
An earlier conclusion prevalent among geologists (and perhaps still retained by some) was, that the Great Oolite limestone of the high grounds of the Northampton district, was identical with the limestone (the Lincolnshire Limestone) which occurs between Kettering and Stamford, and, characterizing the country about the latter town, extends on through Rutland and Lincolnshire into Yorkshire ; that this Limestone was a member of the Great Oolite series ; and that the calcareo-arenaceous slate of Collyweston and Easton, which bases this limestone, was the equivalent of the Stonesfield Slate of Oxfordshire : and this opinion has been maintained notwithstanding the discrepancy indicated by the very dissimilar fossil contents of the two formations and by the anomaly of their relative stratigraphical position.
I hope to be able to show the distinction between these two formations ; to demonstrate, from the succession of beds at different points between Northampton and Stamford, and in the districts about Stamford, that, while the position of the Northampton Lime-
