Keeping in view, then, that the formations into which the geologist has grouped the stratified portion of the earth's crust have a settled and invariable order of occurrence, that each of them contains its own peculiar and characteristic group of organic remains whereby it can be recognised in any part of the world, and that such remains form often the sole test at once of the geologic age and of the origin of the rocks wherein they lie, we may return to the plan above proposed and endeavour to understand the structure of a coal-field. For this purpose it may be well to select one of the northern coal-fields of Britain, since these perhaps display a greater variety in their organic contents, and bear evidence of more diversified changes in their mode of formation than can be seen in those of the south. The strata that compose the coal-basin of Mid-Lothian will probably best suit our purpose, as they are free from the disturbing effects of those igneous intrusions which play so important a part among similar rocks to the north and west. The Mid-Lothian coal-field comprises a mass of stratified beds of sandstone, shale, coal, ironstone, and limestone, the united depth of the whole being above 3000 feet. By reference to the annexed Table it will be seen that the lowest beds of the section are chiefly sandstones and shales, extending downwards to an unknown depth, without any coal that can be profitably worked. These under-strata form the Lower Carboniferous group. Above them comes a middle zone in which the characteristic beds are of limestone, comprising the middle portion or Mountain Limestone of the Scottish Carboniferous rocks. The third and highest subdivision forms the Upper Carboniferous group or true Coal Measures, and constitutes the whole of what is properly the Mid-Lothian coal-field. For the sake of noting some of the remarkable changes exhibited in the character of the rocks, it may be well to begin our survey among the upper beds of the under group. Let us take as our base the famous limestone of Burdiehouse, and work our way upward through the four thousand feet of strata that lie piled above it. VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD.