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A HISTORY OF LEICESTERSHIRE The thickness of these shales is about 200 ft., but it varies somewhat in different places. Exposures in them are very rare, and consequently it is only from artificial excavations that the above classification can be made out. The Upper Lias usually rises in a steep bank above the terrace formed by the Rock Bed ; and produces an undulating district, much covered by Drift, which extends along the eastern side of the county from Market Harborough to Pickwell. North of Edmondthorpe the outcrop is much narrower, and the ground being covered with a thick deposit of Drift these beds are but obscurely seen. The dark shales of the Lias have been mistaken for Coal Measures, which sometimes led in former times to fruitless trials for coal, as was the case at Billesdon Coplow. 1 That the Lias has been laid down in seas of varying depth is indicated by its fossil contents. The passage from the Rhaetic with fragmentary remains to the lower beds of Lias with Ammonites, Saurians, and Fish shows a gradual change from shallow lagoons to an open sea. The sandy beds of the Middle Lias and the basement portion of the Upper Lias indicate the oncoming of shallower water, which again deepened when the main mass of the Upper Lias clay with its abundant Ammonites was laid down. INFERIOR OOLITE The Inferior Oolite, which makes such a fine escarpment just beyond the eastern boundary of the county, is but poorly represented in Leicester- shire. Small patches, however, of the rock are found capping hills at Nevill Holt, Loddington, Robin-a-Tiptoes, and Whatborough. North of Melton Mowbray a projecting spur of the main outcrop comes within the district about Waltham-on-the- Wolds and Croxton Kerrial. It comprises two divisions, the Northampton Sand and the Lincolnshire Limestone. The first of these is further sub-divided into the two horizons of the North- amptonshire Ironstone and the Lower Estuarine Series. THE NORTHAMPTON SAND The Northamptonshire Ironstone is usually a rich ironstone which, when not altered by the percolation of water, is a hard and compact rock of a blue or green colour composed of carbonate and silicate of iron. Under the microscope it is seen to consist of rounded oolitic grains. The rock, when it occurs near the surface and has been exposed to atmospheric influences, exhibits a peculiar cellular structure resembling a collection of oblong boxes. This has been produced by the concentration of hydrated peroxide of iron along the bedding planes and joints, which has split up the rock into roughly rectangular blocks.' This ironstone passes up into brown sands, which are succeeded by white sands with occasional layers of clay and lignite. To these upper beds the name Lower Estuarine Series has been given. The total thickness 1 Life and Letters ofj. B. Jukes, 1871, p. 467. ' Memoirs of the Geol. Surf. : ' The Geology of Rutland,' 1 1 8, 1 34.