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GEOLOGY in its widest interpretation is a History of the Earth. Its object is to enquire into the nature of the changes which the Earth has undergone from the beginning. The alterations which are now taking place on its surface in the continual waste of the land by rain, rivers, and sea; the dispersion and deposition of the material thus removed ; the phenomena of earthquakes and volcanoes; the rising of land in one place, and its sinking in another; the distribution of animal and vegetable life, their growth and decay-all these incessant changes are but the continuation of the Earth's past history which we read in the consolidated mud, sand, and ooze with their included fossilized remains of animals and plants, and in the old volcanic rocks, which together constitute the outer coating or crust of the Earth. Thus the phenomena now produced by causes we can for the most part see and examine have arisen in past times from similar causes, and continued to do so during a period of time so vast that we cannot realise its length. The very beginning we can but dimly picture and speculate upon, and we need not enter upon this subject, as the special province of the geologist is only to deal with the Earth after it was in a fit state to receive and support life, when the proportion of land to water was probably much as it is now, and the climate and physical conditions, though ever varying over the same area during the different geological ages, were subject to the same laws and attended by analogous phenomena. Nowhere do we detect any evidence of abrupt physical change or universal catastrophe: on the contrary, through every epoch we can discern a Uniformity in cause not only in the inorganic, but also in the organic world, accompanied though it be by evidences of Evolution or the gradual development of higher and higher forms of life.

To go back to the very earliest history of the Earth, when it was part of a nebulous mass, would be to trespass upon the region of the Astronomer; and when we consider its latest history, we come upon questions which must be answered by the Geographer and the Archäologist.

In all such questions, however, we must not limit our ideas to what we see in our small tract of the British Islands ; nor must we, in concluding that the physical forces have been the same throughout geological time, suppose that their action has been always of similar intensity to that of which we have definite proof in the present.

The changes we have to deal with are those represented by the crust of the Earth, or that part of it which alone is accessible to human observation. This crust is composed of different rocks which are arranged according to their method of formation into four classes—1. Aqueous, or Sedimentary;' 2. Sub-aërial; 3. Igneous, or Eruptive; and 4. Metamorphic.

1. Aqueous, or Sedimentary Rocks.—These include most rocks deposited under water, as Sand, Sandstone, Clay, Shale, Marl, Chalk, Limestone.

2. Sub-aërial Rocks.These include deposits formed on land, as Peat, Coal, Æolian or Blown Sand.

1 Sometimes called Neptuniun.

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:

When indurated

or con pacted

Lithological constitution,
or cuief Mineral in.







or Greywacke.



Breccia or Angular, sub-angular,
conglomerate and rolled fragments

of all kinds of rocks
Conglomerate Pebbles of hard rocks,
or Pudding- such as limestone,

flint, sandstone,

quartz, quartzite Sandstone Quartz grains someand

times cemented by

calcareous matter : Grit'

fin--grained sili.

ceous rock

Clay and sand

Silicate of alumina

Clay and carb. Jime.
Marlstone Argillaceous lime-

Limestone Carbonate of Jime


Loam or

Slate (when cleaved)


Marble in some in



mud or Ooze

1 The terms grit and sandstone are used very indefinitely. Mr. F. Rntley describes Grit as 'a coarse-grained and somewbat 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 ornamertal character, if capable of being polished, are called Marble.

The AQUEOUS OR SEDIMENTARY and SUB-AËRIAL Rocks may be again classified, according to their method of formation, thus:

1. Mechanically formed. Gravel, Sand, Conglomerate, Breccia, Clay, Shale, Loam, Marl,

some Limestones.

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.

· Partly formed organically.

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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

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