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3. Igneous, or Eruptive Rocks.-These include the rocks which have been intruded as bosses and dykes, or have been poured out as lava-flows, or ashes from volcanic centres, as Basalt, Greenstone, Trachyte, Felstone, Porphyry, Granite, &c.
4. Metamorphic Rocks.-These include rocks which have undergone great alteration by heat and pressure, as Granite? Mica-Schist, Gneiss, some kinds of Marble, Slate, &c.
The arrangement and characters of the AQUEOUS, or SEDIMENTARY rocks will be seen from the accompanying Table:
1 The terms grit and sandstone are used very indefinitely. Mr. F. Rutley describes Grit as a coarse-grained and somewhat coherent, or, at times, a fine-grained and very hard compact sandstone, frequently containing fragments and granules of other minerals beside Quartz, Flint, or Chert.' Any limestone and some other rocks of ornamental character, if capable of being polished, are called Marble.
The AQUEOUS OR SEDIMENTARY and SUB-AERIAL ROCKS may be again classified, according to their method of formation, thus:
1. Mechanically formed.
Gravel, Sand, Conglomerate, Breccia,' Clay, Shale, Loam, Marl,
2. Chemically formed.
Dolomite, Tufa, Iron-stone, Nodules and Septaria, Rock-salt, Flint, Chert, some Limestones, Gypsum.
3. Organically formed.
Some Limestones, Coal, Lignite, Peat.
In all attempts to classify the rocks, we find, however, that hard lines of separation do not exist, as many agencies work together in their formation. Deposits formed by chemical action are mingled with organic remains and with mechanically formed sediment.
Some limestones may be merely eroded granules of preexisting limestone carried mechanically in suspension in water, and ultimately deposited as a sediment. Some may have resulted from the precipitation of carbonate of lime from water holding the bi-carbonate of lime in solution. In this case the deposit may be considered to have a chemical origin. Travertine, calcareous Tufa, and Pisolite, are rocks formed in this manner. The last-named consists of rounded grains like shot or peas, whence the name Pisolite, or Peastone-the finer varieties being termed Oolite or Roe-stone.
Limestones may also at times be composed in great part of the minute calcareous tests or shells of Foraminifera, or of the shells of other organisms; or they may be largely due
1 Fault-rock, or the material filling up the crevices of faults, may be included here.
2 Partly formed organically.
to the secretion of carbonate of lime by Corals. These rocks may be termed of organic origin.
Again, the siliceous matter in beds of chert and in nodules of flint may be due in the first instances to the accumulation of organisms having siliceous structures, but the formation of the beds and nodules themselves is evidently due to inorganic-chemical or physical-agency.
Deposits of very varied origin may be commingled in the neighbourhood of active Volcanoes, the ashes from which are frequently carried to great distances, and may be deposited on the ocean-bed as a sedimentary deposit, together with organic and other accumulations.
To turn to the subject of Igneous rocks, we find that although a study of the volcanic phenomena that are presented to our view at the present day throws considerable light on their former history, yet our opportunities of observation are necessarily limited to the rocks now thrown up at the surface, whereas many of the old igneous rocks, belonging to the various geological epochs, have evidently been formed or intruded at a depth below the surface, and have perhaps never appeared to the light of day, until, in comparatively recent times, disturbances and denudation have together assisted to reveal them.
Igneous rocks are of all ages, and occur as bosses and dykes bursting through and penetrating the stratified rocks. Sometimes they have flowed over a surface upon which other beds were afterwards deposited, so that they are intercalated with these rocks; at others they have forced their way between already hardened beds, and given a false appearance of contemporaneity.
Rocks, too, which were once deposited under water, have, by contact with igneous rocks, or through pressure, lost their original stratified character, and become altered or metamorphic. All rocks, indeed, are in a certain sense
metamorphic, as all have undergone some changes, however trifling they may be, since their formation; but they are not usually termed metamorphic unless their original structure is much obscured.1
The study of rock-masses, of bedding, stratification, and other features only to be observed in the field, is called Petrology; the study of rocks in respect of their mineral composition and microscopic structure is called Lithology.
There can be no doubt that the aqueous rocks, as a rule, were formed and deposited on the ocean-bed in the same way as such rocks are now accumulated--the pebble-beds and sands near the shore, the clayey and marly beds, and the limestones in the deeper water. Gravel, sand, and clay are likewise formed by rivers and glaciers, and all kinds of sediment may be deposited in lakes.
It may be noticed on any sea-coast where there are cliffs, how the ceaseless action of the breakers is ever wearing away the rocks. Great portions of the cliffs are undermined, and masses fall down and are pounded up by the waves into shingle and sand. The finer matter is often carried to some distance from the shore and laid down at the bottom of the ocean. Thus in time great deposits are formed.
Again all rivers bear along with them much material held in suspension, which has been worn away from their banks or carried into them by springs and rivulets, and this they carry into the sea or lake, as the case may be; the heavier materials naturally sinking first, and the finer or lighter particles being carried to the greatest distance.
The stratified rocks occurring one above the other form a series, which is however subject to much modification. We find throughout all time, that while deposits have
1 As the classification of these rocks is given in the chapter on the Igneous and Metamorphic Rocks, it is unnecessary to say more about them here.
been forming very much as they do now in limited areas, the areas have continually changed from dry land to water, and vice versa. The deposits formed have been upheaved, and partly worn away or denuded; fresh ones have been afterwards spread over their worn surfaces; and thus, although there is a certain regularity, the series, as we shall see, is marked by the local absence of many of its members.
Our English and Welsh rocks are mostly of the aqueous or sedimentary kind--rocks deposited under water, and arranged in layers or strata,' whence they are called stratified. And it may be safely assumed that all such beds were originally deposited in an approximately horizontal position, and that any considerable deviation from this, such as is often exhibited by the planes of bedding, is due to subsequent disturbance.
It is true that we have clay, limestone and marl, sandstone, sand and conglomerate, intercalated one with another, occurring at all horizons in the earth's crust, generally harder or more compact the older they are; but it is well known that these rocks are arranged in a certain regular series, distinguished one from another by peculiar mineral characters, and by the occurrence in them of particular fossils, and that the newer the rocks in which these latter are embedded, the more closely are they allied to the forms of life now in existence.
All those who have travelled have no doubt observed the different character of the rocks in different places-in one locality chalk, in another sandstone; here clay, there slate or limestone; but the difficulty is to comprehend the relation that one rock bears to another, and to understand their sequence.
The relations of the different strata have been determined by actual observation of one set of beds superimposed on another in many cliffs, quarries, and railway-cuttings, and by the records of deep mines and wells. When such evidence