The Garden Guide

Book: Gardening Science - Soils, Manure and the Environment
Chapter: Chapter 1: Earths and Soils

Fossiliferous rocks

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1141. Fossiliferous rocks. These are all deposited in strata, and though very frequently some of the series are deficient, what are called the lower rocks are never found above the higher ones ; and in this respect they show a marked difference to the igneous rocks, which seem to be acted upon from time to time by internal fires, and pushed up violently through the strata which have been deposited above them, tearing the stratified rocks asunder, and occasioning those curious faults or breaks which frequently occasion so much trouble and annoyance to miners. In the lowest slate and limestone rocks, a few fossils have been discovered, but a far greater number are found in a series of rocks which are called Silurian from their being best developed in that part of England and Wales which was formerly included in the ancient British kingdom of the Silures. Some of the rocks belonging to this system are found occasionally in comparatively thin strata, which are very much twisted and distorted. Sometimes the intermediate rocks are altogether wanting, and 'the beds resting upon the gneiss, mica-schist, and other old rocks, consist, for the most part, of coarse conglomerate or pudding-stone, evidently made up of the broken fragments of the old granitic rocks, rolled and tossed about for ages in a troubled sea, the hardest stones being rounded into bullet-shaped pebbles by their long and incessant attrition against one another.' (Ancient World, p. 57.) Thus, the rock known as the old red sandstone, which is below the carboniferous system, has often move the appearance of a conglomerate than a sandstone, and the Devonian rocks consist of gritty and slaty beds, the grains of sand appearing to be fragments of white rolled quartz, surrounded by a red peroxide of iron like a varnish, the oxide of iron being an abundant substance in volcanic ejections. The mountain or solid limestone which lies above these rocks, appears to have been the receptacle of great masses of vegetable remains which time has changed into the substance we call coal. This carboniferous limestone forms the underlying rock in the immense tract called the British Coal Field, which extends from Bristol and South Wales to the north and east, forming the limestone and coal districts of the midland and northern districts of England. The immense thickness of these carboniferous strata, which is sometimes two thousand yards and upwards, renders it difficult to comprehend how such enormous masses of vegetable matter can have been accumulated and buried. In some places the mountain limestone is absent, and the coal measures rest immediately on the old rock; and sometimes muddy and sandy beds alternate with one another, and with the coal itself. Some of these appear to be of freshwater origin, and others deposits from the sea, and they generally contain remains of the leaves of ferns and fern-like trees. 'The trees,' says Professor Ansted, 'which, in many cases, contributed largely to the formation of the coal, seem to have been almost entirely succulent, and capable of being squeezed into a small compass during partial decomposition. This squeezing process must have been conducted on a grand scale, both during and after the formation of separate beds, and each bed in succession was probably soon covered up by muddy and sandy accumulations, now alternating with the coal in the form of shale and grit-stone.' (Ancient World, p. 79.) It is a singular fact that in several places in the north of England, the mountain limestone serves as a receptacle for lead, and other metals. Above the coal measures is generally found magnesian limestone, and above this the new red sandstone including various kinds of marl, and also gypsum, and rock-salt. Above this is the lias, Bath, and other oolitic limestone, Portland stone, and various kinds of clay and sand. Above this lies green sandstone, gault, and chalk, the latter being intersected with rows of flints; and above these are the tertiary strata, including the London clay, &c., which are covered by the beds of loose sand and gravel, &c., and by the loose earth on the surface, which is what is usually denominated soil.