called, of volcanic rock, which have burst through the other materials. Such dikes are also observed in the structure of Vesuvius, Etna, and other active volcanos.

There are also other rocks in almost every country in Europe, which we infer to be of igneous origin, although they do not form hills with cones and craters. Thus, for example, we feel assured that the rock of Staffa, and that of the Giant's Causeway, called basalt, is volcanic, because it agrees in its columnar structure and mineral composition with streams of lava which we know to have flowed from the craters of recent volcanos. We find also similar basaltic and other igneous rocks associated with beds of tuff in various parts of the British Isles and also forming dikes, such as have been spoken of; and some of the strata through which they cut are occasionally altered at the point of contact, as if there had been an exposure to the intense heat of melted matter.

The absence of cones and craters, and long narrow streams of superficial lava, in England and many other countries is partly to be attributed to the eruptions having been sub-marine, just as a considerable proportion of volcanos in our own times burst out beneath the sea or the eruption may have been from fissures in the earth's surface. But this question must be enlarged upon more fully in the chapters on igneous rocks, in which it will also be shown, that as different sedimentary formations, containing each their characteristic fossils, have been deposited at successive periods, so also volcanic sand and scoriæ have been thrown out, and lavas have flowed over the land or bed of the sea, or have been injected into fissures, at many different epochs; so that the igneous as well as the aqueous and aërial rocks may be classed as a chronological series of monuments, throwing light on a succession of events in the history of the earth.

Plutonic rocks.-If we examine a large portion of a continent, especially if it contain within it a lofty mountain range, we rarely fail to discover two other classes of rocks, very distinct from either of those above alluded to, and which we can neither assimilate to deposits such as are now accumulated in lakes or seas nor to those generated by ordinary volcanic action. The members of both these classes of rocks agree in being highly crystalline and destitute of organic remains. The rocks of one class have been called plutonic, comprehending all the granites, syenites, and certain porphyries, which are allied in some of their characters to volcanic rocks. The members of the other class are stratified or foliated, and often slaty. They are the crystalline schists, or metamorphic rocks, in which group are included gneiss, micaceous schist, hornblende-schist, statuary

marble, the finer kinds of roofing slate, and other rocks afterwards to be described.

As it is admitted that nothing strictly analogous to these crystalline rocks can now be seen in the progress of formation on the earth's surface, it will naturally be asked on what data we can find a place for them in a system of classification founded on the origin of rocks. It may be stated as the result of careful study that the various kinds of rocks, such as granite and syenite, which constitute the plutonic family, are of igneous or aqueo-igneous origin, and have been formed under great pressure, at a considerable depth in the earth, or sometimes, perhaps, under a certain weight of incumbent ocean. Like the lava of volcanos, they have been melted, and have afterwards cooled and crystallised, but with extreme slowness, and under conditions very different from those producing such volcanic rocks. Hence they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth's surface, or beneath seas of inconsiderable depth. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases and steam give rise in ordinary lava.

Metamorphic crystalline rocks.-The last great division of rocks includes the crystalline strata and slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other classes. They rarely contain any pebbles, or sand, or scoriæ, or angular pieces of imbedded stone, or traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corresponding in form and arrangement to those of sedimentary formations, and are therefore said to be stratified. The beds sometimes consist of an alternation of minerals varying in colour, composition, and thickness, precisely as we see in stratified fossiliferous deposits. According to the Huttonian theory, which I adopt as the most probable, and which will be afterwards more fully explained, the materials of these rocks were originally deposited from water in the form of sediment, but they were subsequently so altered by heat, chemical action, and pressure, as to assume a new texture, and often mineral composition. It is demonstrable, in some cases at least, that such a complete conversion has actually taken place, fossiliferous strata having exchanged an earthy for a highly crystalline texture for a distance of a quarter of a mile from their contact with granite. In some cases dark limestones, replete with shells and corals, have been turned into white statuary marble, and hard clays, containing vegetable or other

remains, into slates called mica-schist or hornblende-schist, every vestige of the organic bodies having been obliterated.

Heated water permeating stratified masses and great pressure have no doubt played their part in producing the schistose and foliated texture and other changes, and it is clear that the transforming influence has often pervaded entire mountain masses of strata.

In accordance with the hypothesis above alluded to, I proposed in the first edition of the Principles of Geology' (1833) the term 'Metamorphic' for the altered strata, a term derived from μerá, meta, trans, and poppý, morphe, forma.

This metamorphism may be local or regional, and was more intense in the earlier geological periods than subsequently.

Hence there are five classes of rocks considered in reference to their origin-the aqueous, the aerial, the volcanic, the plutonic, and the metamorphic. In the course of this work it will be shown that portions of each of these five distinct classes have originated at successive periods of the world's history.

The aqueous and aërial classes have been produced by energies acting on the outside of the globe, and the volcanic and plutonic have been produced by internal energies. The metamorphic rocks have had a double origin, so far as their producing agents are concerned.

The term plutonic applies to the crystalline rocks, like granite and syenite, which differ in degree from the volcanic rocks. The term 'hypogene' was proposed in the 'Principles of Geology' (ed. i. vol. iii.), a word derived from úzó, under, and yivoμai, to be, or be born, to indicate that the crystalline plutonic rocks are nether formed rocks, and which have not assumed their present form and structure at the surface. They never simply repose on volcanic or sedimentary rocks, and can be traced beneath everything, and they underlie all other rocks.

But metamorphic rocks of the advanced type of gneiss and mica schist do not appear to have necessarily been formed at great depths, or under the conditions which granite required for its genesis. Hence the term hypogene is hardly applicable to them. The term hypogene action has lately been aptly employed by Professor A. Geikie1 to express the changes within the earth caused by original internal heat and chemical action, of which the intrusion of granites as eruptive rocks and metamorphism on a grand scale are examples.

From what has now been said, the reader will understand that each of the great classes of rocks may be studied under two distinct points of view; first, they may be studied simply as 4 Text-book of Geology (1882), p. 196.

mineral masses deriving their origin from particular causes, and having a certain chemical composition, form, and position in the earth's crust, or other characters, both positive and negative, such as the presence or absence of organic remains. In the second place, the rocks of each class may be viewed as a grand chronological series of monuments, attesting a succession of events in the former history of the globe and of its living inhabitants.

I shall accordingly proceed to treat of each class of rocks; first, in reference to those characters which are not chronological, and then in particular relation to the several periods when they were formed.

CHAPTER II.

AQUEOUS ROCKS-THEIR COMPOSITION AND FORMS OF

STRATIFICATION.

Mineral composition of strata-Siliceous rocks-Argillaceous-CalcareousGypsum-Loess-Coal-Soil-Ice-borne rocks-Forms of stratificationOriginal horizontality-Thinning out-Diagonal arrangement-Ripple mark.

IN pursuance of the arrangement explained in the last chapter, we shall begin by examining the aqueous and aërial or sedimentary rocks, which are for the most part distinctly stratified, and often contain fossils. We may first study them with reference to their mineral composition, external appearance, position, mode of origin, organic contents, and other characters which belong to them as sedimentary formations, independently of their age, and we may afterwards consider them chronologically or with reference to the successive geological periods when they originated.

I have already given an outline of the data which led to the belief that the stratified and fossiliferous rocks were originally, with rare exceptions, deposited under water; but, before entering into a more detailed investigation, it will be desirable to say something of the ordinary materials of which such strata are composed. These may be said to belong principally to three divisions-the siliceous, the argillaceous, and the calcareous, which are formed respectively of flint, clay, and carbonate of lime. Of these, the siliceous are chiefly made up of sand, or flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter with certain proportions of aluminous earth; and, lastly, the calcareous rocks or limestones, of carbonic acid and lime, sometimes with carbonate of magnesia.

Siliceous and arenaceous rocks. To speak first of the sandy division: beds of loose sand are frequently met with, of which the grains consist entirely of silica, which term comprehends all purely siliceous minerals, as quartz and common flint.1 The siliceous grains in sand are usually more or less rounded, as if by the action of running water. Sandstone is an aggregate of such grains, which often cohere together without any visible cement, but more commonly are bound together by a slight quantity of siliceous or calcareous matter, or by oxide of iron, clay or felspar.

Amongst the siliceous rocks, the result of deposition of fragments of previously existing formations, are gravels, river sands, and sandstones, which may be flagstones, which are thin bedded and split along the lines of stratification; freestones, which are not divided by laminæ, and grits, which have the grains large and visible to the eye; breccias, or angular masses of sandstone more or less included in a common uniting material; conglomerates, with rounded pebbles included in a calcareous or siliceous matrix; arkoses or granitic sandstones; micaceous sandstones with thin silvery plates of mica arranged in layers paralled to the planes of stratification, giving a slaty texture to the rock. Greywacke, a hard grey or dark coloured rock, is a collection of rounded or sub-angular grains of quartz felspar, slate, &c., cemented by a paste which may be siliceous, clayey, felspathic or calcareous, and even anthracitic. This rock, often ripplemarked and sun-cracked, accumulated in running water in ancient geological periods. In nature there is every intermediate gradation from perfectly loose sand to the hardest sandstone.

Argillaceous rocks.-Clay, strictly speaking, is a mixture of silica or flint with a large proportion, usually about one-fourth, of alumina; but in common language, any earth which possesses sufficient ductility, when kneaded up with water, to be fashioned like paste by the hand, or by the potter's lathe, is called a clay; and such clays vary greatly in their composition, and are, in general, nothing more than mud derived from the decomposition or wearing down of rocks. The purest clay found in nature is porcelain clay, or kaolin, which results from the decomposition of granite. Shale has also the property, like clay, of becoming plastic in water: it is a more solid form of clay, or argillaceous matter, condensed by pressure. It always divides into more or less regular laminæ.

Flinty slate, or Lydian-stone, and clay-slate are forms of clay which have undergone changes from the infiltration of chemical solutions. Mud-stone is a sandy, clayey rock. Fullers' earth is a

1 For a description of all usual rock-making minerals see the Appendix.