land and most thermal waters; has been found in a pulpy state within basalt; forms the tabasheer found in the cavities of the bamboo, and the thin pellicle or outer covering of canes, reeds, grasses, &c.; and siliceous concretions are common in the fruits and trees of the tropics. All these facts point to a very general diffusion of silex in a state of solution; and whatever may have caused its abundance in the waters during the deposition of the upper chalk, there can be little doubt respecting the mode in which it has been collected around the organic remains of these early seas. EXPLANATORY NOTE. THE ORIGIN OF CHALK, so different in its texture and appearance from all other limestones, has given rise to many hypotheses. "There appears no evidence," says Mr Brande, "of its having been deposited from chemical solution; but, on the other hand, it bears marks of a mechanical deposit, as if from water loaded with it in fine division. And upon this principle, some gleam of light may perhaps be thrown upon the enigmatical appearance of the flints; for it is found, that if finely-powdered silica be mixed with other earthy bodies, and the whole diffused through water, the grains of silica haye, under certain circumstances, a tendency to aggregate into small nodules; and in chalk, some grains of quartz are discoverable." There can be little doubt that such has been the original state of chalk, from whatever source derived; for, without the supposition that the calcareous particles were diffused through the waters in which it was deposited, it were impossible to account for almost any of the phenomena connected with it as a formation. But while such has evidently been the origin of the great mass of the chalk rocks, it does not preclude the chemical agency of springs, or the organic efforts of secreting animalcules. All other limestones in the crust of the earth point to a complex formation, in which mechanical, chemical, and organic agencies have been concerned; and it is but reasonable to suppose that chalk is the result of similar forces. BELEMNITES (Gr., belemnon, a dart)-a genus of fossil-chambered shells, perforated by a siphuncle, and so called from their straight dart-like form. Unlike other chambered shells, they were internal; that is, enclosed within the animal like the pen of the squid and cuttle-fish. Many of these belemnites are of great size, showing the gigantic nature of the cephalapods to which they belonged. Being long, straight, and conical, they are commonly known by the vernacular names of "thunder stones" and "thunder bolts." реда Nummiulio/sofago. piece! - money vary crown much I microscofive litt in sens e a -ness/accumulated 15 2 dary / U ours Intiary strata. / Forc / az &'s entire bulk ou mountains/ge. in 2 dory strata, Cretacious formione, Confettura Pyrenus. — TERTIARY STRATA, 239. THE TERTIARY SYSTEM comprises all the regular strata of limestone, marl, clay, sand, and gravel which occur above the chalk. Before the labours of the celebrated Cuvier and M. Brogniart, these beds were regarded as mere superficial accumulations, not referrible to any definite period. Now, however, they are recognised as constituting a distinct formation-differing from the cretaceous not only in its mineral composition, but in the higher order of organisms which it contains, and from the superficial sands and clays, in being regularly stratified, and in imbedding the remains of animals distinct from existing races. In general the strata are loosely aggregated, are of no great thickness, and present appearances which indicate frequent alternations of marine and fresh-water agencies. Thus, marine remains are found in some beds, while others contain exclusively land animals and plants, and freshwater shells. The whole suit being less consolidated than any of the secondary systems, and containing plants and animals approaching to existing forms, it presents a freshness of aspect which serves to distinguish it from older deposits; at the same time the regularity of its deposition prevents it from being mistaken for any mere alluvial accumulation. In general it occupies very limited and detached areas, as if it had been formed in shallow inland seas and estuaries, to which the waters of the ocean at times had access, and where at other periods fresh-water inundations prevailed. Another essential difference between the tertiary and the more ancient formations consists in the fact, that the latter maintain a wonderful uniformity in their composition and character all over the globe; whereas the former present almost as many distinctions in composition as there are areas of deposit. For this reason it is impossible to give a description applicable to all tertiary strata; those of England and France, however, may be taken as types sufficiently characteristic. 240. Respecting the composition of the system, arenaceous and argillaceous beds may be said to prevail, with interstratified limestones, calcareous grits, and marls. The arenaceous members are either pebbly conglomerates of a rusty yellow, or sands little indurated and variously tinted by the oxide and silicate of iron. The sands are seldom sufficiently consolidated to form sandstones; and the conglomerates are often mere layers of rolled pebbles, without any cementing matrix. The argillaceous beds also present many varieties; some being turbed by inundations from the land, otherwise ferns and other plants, and not fuci, would have been found in the sandy strata." It is true that the evidence respecting the climate, and other conditions of the cretaceous era, is still imperfect; but the recently-discovered remains of the highest order of mammalia (quadrumana, or monkey tribe) point to a tropical climate; and this fact, taken in conjunction with the occurrence of cycadites, seems to establish a temperature little different from that which prevailed during the wealden epoch. The foregoing engraving represents a few of the more characteristic fossils belonging to the sys tem. 234. Igneous rocks are nowhere associated with the chalk in England; but basalt and other traps break through and overlie the strata in the north of Ireland-the Giant's Causeway presenting one of the finest examples of this connexion. In the Pyrenees, cretaceous strata are said to be in contact with granitic rocks; but, generally speaking, the system has escaped with fewer displacements by igneous agency than any of the earlier formations. As has been stated, where chalk comes in contact with igneous discharges, the heat has rendered it hard and crystalline like primary marble. The same effects have been produced by enclosing pounded chalk in an iron tube, and subjecting it to the heat of a furnace. 235. The geographical extent of the system is limited, when compared with earlier formations. It is pretty extensively developed in the south and south-east of England, filling up the hollows and basins left by the oolite and lias. It appears in the north of Ireland overlaid by basalt and other trap rocks; but is unknown in Wales or in Scotland. It is spread over wide areas in France and Germany; and is found about Dresden, in the Alps, Carpathians, and Pyrenees. According to Professor Rogers, it occupies a vast area in the North American states; and, by recent accounts, has been detected in the western river-plains of South America. 236. The physical aspect of chalk districts is easily distinguished by the smooth flowing outline of the hills and valleys. Here there are no rugged and lofty peaks, as in the earlier formations; no tabular-looking escarpments, as in the lias and oolite; but easy undulations, forming in their extent the wellknown "wolds" or "downs" of southern England. These downs are characterised "as covered with a sweet short herbage, forming excellent sheep pasture, generally bare of trees, and singularly dry even in the valleys, which for miles wind and receive complicated branches, all descending in a regular slope, yet are frequently left entirely dry; and, what is more singular, contain no channel, and but little other circumstantial proof of the action of water, by which they were certainly excavated." Chalk districts thus possess great amenity and rural beauty, and are as yet but little broken up by the enterprise of modern agriculture. 237. The minerals of commerce derived from the system are by no means numerous. Chalk is used for many purposes in the arts and in agriculture; it furnishes polishing paste, and the well-known whiting of the painter. Beds of fuller's earth occur in the lower green-sand, and in some districts the more indurated strata of the group produce a rough buildingstone. Flint is one of the most valuable products of the system; furnishing material for the manufacture of china and porcelain, flint-glass, and gun-flints—the latter having been in universal use before the invention of the percussion cap. 238. The formation of flint, within a mass so different in composition as chalk, is still in some respects an unsolved problem in geology. It occurs in nodular masses of very irregular forms and variable magnitude; some of these not exceeding an inch, others more than a yard in circumference. Although thickly distributed in horizontal layers, they are never in contact with each other, each nodule being completely enveloped by the chalk. Externally, they are composed of a white cherty crust; internally, they are of gray or black silex, and often contain cavities lined with calcedony and crystallised quartz. When taken from the quarry they are brittle and full of moisture, but soon dry, and assume their well-known hard and refractory qualities. Flints, almost without exception, enclose remains of sponges, alcyonia, echinida, and other marine organisms, the structures of which are often preserved in the most delicate and beautiful manner. In some specimens the organism has undergone decomposition, and the space it occupied either left hollow, or partially filled with some sparry incrustation. From these facts, it would seem that flints are as much an aggregation of silex around some organised nucleus, as septaria (par. 192) are aggregations of clay and carbonate of iron. This is now the generally received opinion; and when it is remembered that the organisms must have been deposited when the chalk was in a pulpy state, there can be little difficulty in conceiving how the silex dissolved through the mass would, by chemical affinity, attach itself to the decaying organism. Chalk is composed of carbonate of lime, with traces of clay, silex, and oxide of iron; flint, on the other hand, consists of 98 per cent. of pure silex, with a trace of alumine, oxide of iron, and lime. Silex is quite capable of solution: it occurs in the hot-springs of Ice almost pure laminated clay of a dull blue colour, others of a brownish tint, with a slight admixture of sand, while many pass into marls more or less calcareous. None of these clays are so compact as to form shales; indeed lamination is more frequently absent than otherwise, there being nothing except their fossils and associated beds to distinguish them from the clays of subsequent alluvial valleys. The calcareous layers are still more varied in their composition and aspect, and bear no resemblance to the indurated half-crystalline limestones of older formations. The marine limestone of the Paris basin is of a coarse sandy texture; that of Austria a rough coralline rock the fresh-water beds near Weimar are hard and compact; those of other districts are soft, marly, and full of shells. In some localities marls are so calcareous as to be used as limestones, while in others they pass into soft friable clays. From this extreme diversity of composition, it is evident that many agencies have been concerned in the deposition of the tertiary system, and that most of them have been of a local character, producing results not differing widely from those of the present day. 241. The succession of strata is no less varied than their mineral composition. As at the present day distant rivers are depositing different sorts of material at one and the same time, so in distant tertiary basins different strata variously succeed each other. Luckily, none of the deposits are of great thickness, and as they have been closely examined for the sake of their fossils, the alternations of the beds have been pretty accurately ascertained. The following is a descending section of the Paris basin, according to Cuvier and Brogniart :— 5. UPPER FRESH-WATER GROUP-marls, marly sands, shelly limestone, and siliceous or burr limestone. 4. UPPER MARINE GROUP-marls, sands and sandstones of a white or ochraceous colour, and loosely aggregated; thin layers of limestone. 3. LOWER FRESH-WATER-marls, gypsum (sulphate of lime), with bones of animals, and siliceous limestones. 2. LOWER MARINE-consisting principally of a coarse sandy limestone (calcaire grossier), with calcareous marls and layers of greenish sand. 1. PLASTIC CLAY GROUP-consisting of bluish plastic clays, with layers of sand, beds of lignite, and rolled pebbles. Supposed to be of estuary origin. Although a very different succession takes place among the tertiaries of the south of England, yet there is sufficient resemblance in the position and aggregation of their strata, as well as in their organic remains, to establish the fact, that they belong to the same epoch as the rocks of the Paris basin. The |