A HISTORY OF DERBYSHIRE shire has increased considerably, and although at present greater than that of Nottinghamshire, Mr. Stokes thinks that it has almost reached its limit and will in turn be exceeded in the neighbouring county. At the commencement of the nineteenth century the coal raised in Derby- shire amounted to about 270,000 tons, whilst during the last year of the century it had reached 1 5^ million tons. In addition to the main coalfield there are two others of small area which deserve a brief mention. That in the southern portion of the county is a continuation of the Leicestershire field. The boundary is very irregular. It enters the county near Calke Abbey, runs nearly to Ticknall, and in a zigzag line to Bretby colliery. These Coal Measures dip south until close upon the county boundary, where they are overlain by Triassic rocks. The coal of this district, the principal seam of which is called the main coal, is generally used for manufacturing and steam purposes. The north Derbyshire coalfield is a continuation of the Lancashire and Cheshire coal basin. It covers only a small area of the county. Four seams are worked, but no large amount of coal is raised. It is mostly employed for local purposes, and is too inferior in quality and too far from a railway to compete with the coal raised in the east Derbyshire coalfield. Clay ironstone is found either in nodules or in beds amongst the Coal Measure shales. It is in the form of a carbonate of iron mixed often with argillaceous and silicious material. Though it was once largely worked in the county its place has now been taken by the North- amptonshire ore, which can be delivered more cheaply at the Derbyshire furnaces. The fossils of the Coal Measures are indicative of estuarine or brackish water conditions, with a land flora alternating with layers con- taining marine fossils. The presence of the numerous seams of coal and beds of carbonaceous shale point to the profusion of vegetable growth during that part of the carboniferous period when the Coal Measures were being formed. The flora, consisting of some hundreds of forms, has only distant representatives to-day in the tree ferns of tropical swamps and jungles and the horsetails and club mosses of temperate regions. The seams of coal are composed of compressed and mineralized remains of this vegetation. The vegetable matter becomes decomposed, gives off gases, passes through states similar to those of peat and lignite, and is finally transformed into coal. Some coals were undoubtedly formed on dry land or in swampy marshes. That such was the case is shown by the uniformity of character and thickness which a seam of coal often maintains over a considerable area, and by the fact that fossil trees are now found in the Coal Measures in the position in which they grew. A coal seam generally rests on an under-clay or shale or gannister, called ' seat earth ' by the miners. The fossil trees called sigillaria are found erect in the coals with their roots or stigmaria penetrating the under clay. These 24
