The Garden Guide

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

The crust of the earth

Previous - Next

1138. The crust of the earth is all that is attempted to be described by geologists, as nothing certain is known of the earth's internal structure ; and this crust consists partly of rocks which at some distant period have been in a state of igneous fusion, and partly of earthy or mineral matters, which have been held in suspension by water, and afterwards deposited in strata or layers. Rocks of the first kind are sometimes called crystalline or non-fossiliferous, because their structure is crystalline, and they contain no fossils; and sometimes igneous or unstratified, because they bear strong marks of having been subjected to intense heat, and because they are not deposited in distinct strata or layers. The other class of rocks, that is, those which are deposited in strata or layers, are of various kinds. The oldest are evidently formed from particles washed off the crystalline rocks, and deposited when the water in which they were held in suspension was removed; and hence these are sometimes called sedimentary crystalline rocks. Above these lie rocks of various kinds of slate : these form what were formerly called transition rocks, or the upper Grauwacke system, but which are now the Silurian rocks of Sir Roderick Murchison. These rocks abound in fossil remains, but chiefly of corals, Encrinites, and other invertebrated animals. The whole of these rocks were included in the primary strata of the older geologists, and together with the old red sandstone and carboniferous series form what is now considered to belong to the older or Palï¾µozoic period. Above these lies the new red sandstone. Higher still are the lias, the various kinds of oolite, and the Wealden clay; and above these lie the rocks of the cretaceous system, containing green sand, gault, and chalk. These are all the rocks included in the secondary strata, and they all contain fossil remains belonging to extinct species. The tertiary strata consist of the plastic and London clays, and Suffolk crag, These rocks contain numerous fossils, some of which are identical with existing species ; and above them lies only the gravel and what is generally called surface soil, formed by the deposit of river sediments, and the decay of vegetable and animal substances.