Page:VCH Leicestershire 1.djvu/38

 A HISTORY OF LEICESTERSHIRE At Whitwick Colliery and as far south as Ellistown the Coal Measures are overlaid by a sheet of basalt, which was poured out in a molten state, and has charred the measures with which it came in contact. Although the Coal Measures were probably originally laid down over the country to the east of the Charnwood Hills they were swept away in pre- Triassic times ; and there is evidence, from several boreholes that have been made, that the red measures rest directly upon highly inclined beds of a much older date. 1 The physical history of the Carboniferous period is clearer than that ot those that went before, from the fact that the record of this epoch is more complete, and there are more reliable data to work upon. During the early part of this period it is probable that an inland sea existed to the north, extending along what is now the western borders of Derbyshire and York- shire, to the south of Scotland. In this depression was deposited the thick mass of limestone that covers this district ; while in the extreme north and south, where the old land surfaces seem to have principally existed, a greater amount of muddy or sandy matter was brought in. From the small thickness of limestone that occurs in this part of the area it is probable that it was near the southern margin of this sea ; while further proof that the water was comparatively shallow is afforded by the dolomitic character of much of the limestone. After a time a change took place and deposits of mud and sand, which had at first been confined to the neighbourhood of the shore, extended themselves over nearly the whole of this marine basin, and formed the shales, sandstones, and grits that overlie the limestone. Later the sea became con- verted into a freshwater lake or large estuary, while the further filling up of the water or the elevation of the land gradually produced low swampy flats intersected with land surfaces, on which vegetation quickly sprang up, and furnished the material for beds of coal. During this later period constant oscillation of level took place, producing the frequent alternations of shale and sandstone, latterly with coal seams, which characterize the Coal Measures. PERMIAN In this district there is a marked unconformity between the Coal Measures and the succeeding deposits of Red rocks, the lowest of which have been classed as Permian. These rocks belong to a special series of strata, which has been distinguished by Professor Hull as the ' Salopian type ' of British Permian. 8 There is, however, no proof that they are of Permian age ; they may represent the commencement of the Trias epoch, and it is very doubtful whether the separation should be maintained. They are com- posed of coarse angular breccia with occasional beds of marl and sandstone. The breccia is usually angular, especially towards the south, and is composed of fragments of Carboniferous rocks, volcanic ash and other igneous rocks, slates, feldspathic grits and quartzites, embedded in a sandy calcareous matrix. 8 The fragments do not appear to have come from any distance, and most of 1 Details of these borings are given in the Memoirs of the Geol. Surv. : ' The Geology of the Country near Leicester.' ' Memoir, tf the Geol. Surv. : 'The Triassic and Permian Rocks of the Midland Counties of England,' 11. rown, 'The Permian Rocb of the Leicestershire Coalfield,' Quart. Journ. Geol. Soc. xlv, 24. 8