and one or two others, are found native in nuggets, pellets, plates, and thread-like branches; the majority of the metals occur as ores-that is, as oxides, sulphides, carbonates, &c., as shown in the tabulation of mineral groups, p. 15, 16. Most of these ores are found in veins associated with sparry matter, as calc-spar, fluor-spar, quartz, baryta, &c., which form the veinstone, gangue, or matrix ; a few only occur as stratified deposits.

II. CHRONOLOGICAL ARRANGEMENT OF ROCK-FORMATIONS.

It is not enough, however, to determine merely the posi tions, structure, texture, and composition of rocks; the geologist must endeavour to ascertain their relative ages—that is, their succession in time and sequential place in the earth's crust, so as to be able to map out their respective areas, their extent, thickness, and abundance. In this task he is mainly aided by three considerations - superposition, mineral composition, and fossil remains. In any succession of deposits it is obvious that the lowest must be the oldest, and that those above will take their places in chronological order. It is also for the most part true that the older and deeper strata will have undergone a higher degree of internal or mineral change through pressure, chemical replacement, and other metamorphosing agents. And it has been further ascertained that the older or deeper any rock-formation is, the more widely do its organic remains differ from existing genera and species. Guided in his determination by these and similar truths, the geologist has been enabled to arrange the stratified rocks in chronological sequence-that is, into formations, groups, and systems or life-periods, from the deposits now taking place in existing waters to the deepest or most ancient in the earth's crust, and about whose nature and origin he can reason with something like certainty. Having determined the relative ages of the stratified rocks, he also attempts a similar arrangement of the unstratified or igneous, being guided in this attempt by the strata through which they pass, by the fragments of other rocks they may enclose, and lastly, by the manner in which they intersect and overlie each other.

Classification of Stratified Deposits.

In this classification of stratified deposits, the geologist understands by a formation any series of strata that has been deposited continuously in the same area, be that lake, estuary, or sea; and hence he speaks of lacustrine, estuarine, and marine formations. By a group he embraces such strata as have several lithological and palæontological features in common, though they may be partly of fresh-water and partly of

marine origin. And under a system or life-period, he includes such formations and groups as present the same general facies of fossil remains—that is, such groups as are characterised by the presence of the majority of the same plants and animals. Abiding by these principles, modern geologists have arranged the stratified rocks of the crust, and especially those of Europe, as in the annexed tabulation-the terms Primary or Paleozoic (ancient life), Secondary or Mesozoic (middle life), and Tertiary or Cainozoic (recent life), being analogous to the subdivisions of human history into ancient, medieval, and modern :

In further explanation of the preceding synopsis, it may be stated that the Post-Tertiary System consists in the main of clays, gravels, sands, peat-mosses, marls, coral-reefs, foramini feral muds, and other accumulations, which are still forming, or which have been formed within a comparatively recent period, in lake-basins, river-valleys, estuaries, and along the shores as well as in the depths of the ocean. They imbed, in a sub-fossil state, the remains of plants and animals still living, though often removed from, or extinct in, certain localities which they once inhabited. In volcanic districts they are associated with lava, scoriæ, and other igneous ejections; and, generally speaking, they occupy low-lying tracts, and constitute the surfaces of valleys, plains, and other alluvial expanses.

Immediately underlying these recent deposits, there occur over the northern hemisphere (down to the 40th parallel of latitude or thereby) thick accumulations of clay and gravel, imbedding huge water-worn blocks or boulders; and as these seem to point to a time when large areas of the northern hemisphere were under ice, or subjected to the drift of icebergs that dropped their burdens of clay, gravel, and boulders on the then submerged surface, this period is generally known as the Boulder, Northern Drift, or Glacial Epoch, and holds a place intermediate between the Post-Tertiary and Tertiary Systems. For the most part, these glacial accumulations are destitute of organic remains, but in some of the upper and laminated clays, shells, star-fishes, bones of birds, seals, and whales make their appearance, and these are all strictly of boreal species.

The Tertiary System consists, in general terms, of clays, sands, gravels, limestones, marls, and lignites, or beds of woodcoal, and occupies well-defined areas (Basins), as if these at one time had been extensive fresh-water lakes, estuaries, and inland seas. The fossils imbedded in tertiary strata, though closely allied to existing genera and species, are in most instances extinct, and point to conditions of climate and distributions of life very different from that at present prevailing. The igneous rocks associated with them are lavas and basalts, the products of volcanoes long since extinct, or now but partially active. Undulating lowlands may be said to constitute the physical features of tertiary tracts-the basins of London, Paris, and Vienna, and the upper pampas of the La Plata, being typical examples.

The Cretaceous or Chalk System consists, as its name implies, of thick beds of chalk or soft marine limestones, associated with sand, sandstones, clays, and in some localities with beds of coal and lignite. The fossils belong almost wholly to extinct species; and even where the chalk beds are wanting, the

other strata are so replete with the characteristic remains of the system (sponges, foraminifera, sea-urchins, shell-fish) that there is generally little difficulty in recognising it. The associated igneous rocks are chiefly basalts and greenstones; and the physical features of the system may be said to be low rounded hills (like the "Downs" of Kent, Surrey, and Sussex), with dry intermediate depressions (coombs) where the chalks and sand prevail, and flat fertile vales, where the rich fossiliferous clays come to the surface.

The Oolitic or Jurassic System consists largely of limestones, alternating with calcareous clays, sandstones, bituminous shales, and, in some districts, of beds of ironstone and workable coals. It derives its former name from its peculiar limestones or roestones (Gr. oon, egg; lithos, stone), which have a minutely concretionary texture; and its latter from its extensive development in the Jura Mountains. Paleontologically, it is characterised by its cycadaceous plants and tree-ferns, by its abundant marine fauna (corals, bivalves, and nautiluslike ammonites), and by its huge aquatic and terrestrial reptiles. The unequal weathering of its harder limestones, soft clays, and shales, confers on the oolitic landscape that succession of long undulations so noticeable in that broad belt of country which stretches from Yorkshire on the north-east, to Dorset on the south-west of England.

The Triassic and Permian Systems, which were formerly considered as a single system under the name of the New Red Sandstone, consist in the main of soft reddish (sometimes pebbly) sandstones, yellowish magnesian limestones, and variegated clays and marls, with occasional deposits of rock-salt and gypsum. The lower portion, being largely developed in Perm in Eastern Russia, has given rise to the term Permian; and the upper, consisting in Germany of three well-marked members (sandstones, limestones, and marls), has received the name of Trias, or triple group. The fossil remains of these systems differ widely-those of the Permian being closely allied to the Carboniferous flora and fauna, and therefore Paleozoic; while those of the Trias are Mesozoic, and consist of marine organisms, with footprints of birds and amphibian reptiles. The physical features of the New Red Sandstone are by no means decided—the limestones and harder sandstones forming inconspicuous hills and ridges, the softer clays and marls being worn into vales and expanses, of a flat, moist, and retentive character, better fitted for pasture than for cornculture, and of which Cheshire, in our own country, may be taken as a typical example.

The Carboniferous System, so called from its yielding the main supply of coal (Lat. carbo, coal) in Europe and America, consists of sandstones, shales, clays, limestones, ironstones, and coals in frequent alternations, as if they had been deposited for ages in seas and estuaries, subjected to repeated subsidences and elevations. The fossils of the system are abundantly marine, estuarine, and terrestrial, and all of palæozoic forms-the most notable being that excess of vegetable growth that went to the formation of numerous seams of coals. With the exception of the trap hills (greenstones, basalts, amygdaloids, &c.) that intersect the system in some localities, and those bold cliffs and scars of limestone (mountain limestone) so characteristic of Yorkshire and Derbyshire scenery, there is little attractive in the physical features of the coal-formation, monotonous moorlands of cold retentive soil (Northumberland, Lanark, Linlithgow) being a common occurrence in the geography of the system. Though superficially unattractive, it is rich in mineral and metallic products-coals, limestones, fire-clays, building-stones, ironstones, and ores of lead, zinc, silver, and antimony, being among its most important contributions to modern industry and civilisation.

The Old Red Sandstone or Devonian System (from Devonshire, where a portion of it is typically developed) consists in the main of reddish sandstones, conglomerates, flagstones, and shales, with subordinate beds of limestone. In Devon

the fossils are chiefly corals, shells, and other marine exuviæ; in Hereford and Scotland, crustacea and fishes prevail. Thrown into many irregularities by trap (often felstone) eruptions, the physical features of the Old Red Sandstone are usually varied and picturesque, and in general its slopes are dry and of moderate fertility. The larger portions of Devonshire, Hereford, Perthshire, and Forfar, as well as of the south of Ireland, may be taken as typical areas of Old Red Sandstone.

The Silurian System (so called from its typical development in that district of Wales anciently inhabited by the Silures) consists of numerous slaty or hard shaly beds, with sandstones, grits, and intercalated limestones. As in all the older and deeper-seated formations, there is a tendency to crystalline texture, and these are not unfrequently traversed by metalliferous veins tin, copper, silver, and gold. Its fossils are eminently marine, and consist almost wholly of the invertebrate orders (corals, shell-fish, and crustacea), few fishes being found in its strata, and these only in the upper portions of the system. Flanking and often borne up by the older granitic hills, the physical features of the system are frequently irregular and