the silurian implies the agency of the coral animal, precisely similar to that now going forward in the Pacific. Here, then, we have a condition of the world with hills and valleys, rivers and seas-the atmospheric agents acting upon the cliffs and precipices; the aqueous also degrading the rocks, transporting the material, and depositing it along the shores of seas, whose waters gave birth to corals, shell-fish, and fishes. Fine clayey silt formed clay-slate; sand and other mud slaty sandstones; gravel grauwacke conglomerate; and coral polypes beds and reefs of limestone. We have no evidence of terrestrial life; and the necessary inference is, that the conditions of the world did not then permit of its being called into existence. 166. The igneous rocks associated with the transition series are granite, serpentine, porphyry, greenstone, varieties of trap, and mineral veins. Indeed there is scarcely a development of the clay-slate or grauwacke systems without associated granitic rocks; and the greater portion of the silurian strata are thrown into inclined and contorted positions by the same agency, while effusions of trap make their appearance among the latter strata. Perhaps the most extensive and gigantic efforts of volcanic power were exhibited at the close of this period; and there is abundant proof that all the principal mountain chains in the world were then upheaved. The Grampian and Welch ranges, the Pyrenees, Hartz mountains, Dofrafelds, Uralian, Himmaleh, Atlas range, Mountains of the Moon, and other African ridges, the Andes, and Alleghanies, all seem to have received their present elevation at the close of the transition period. From this fact, the student will more readily perceive how the primary and transition strata should be thrown into highly inclined and contorted positions; how they should be traversed by so many dykes and mineral veins; and how slaty cleavage, and other alterations by heat, should have taken place. 167. The extent of country over which the clay-slate, grauwacke, and silurian systems are spread, must be sufficiently indicated by the mention of the principal mountain ranges in the world, from whose sides and flanks their strata slope away for many leagues on either side. 168. The geographical features of transition districts are bold and mountainous, and are well illustrated by the characteristic scenery of Wales, the Cumberland Lakes, and the Scottish Highlands. "Supported by granite," says Professor Phillips, "and mixed with igneous masses, the slaty rocks of the English lakes rise to more than 3000 feet in height, and present a variety of outline, and intricacy of combination, x5 The presence of reins of copper to is dependent on the 90 reher in the vicinity, GEOLOGY. W grows rocks & as 169. The economic uses to which the minerals of these xb SILURIAN--a term invented by Mr Murchison to designate those calcareous and argillaceous beds which lie between the grauwacke and old red sandstone. The word is derived from Silures, the name of an ancient tribe who inhabited that district of country between England and Wales where these rocks are very clearly developed. PYRITES-a mineral composed of sulphur and iron-sulphuret of iron. It is usually of a brass-yellow colour, brilliant, and crystallised. Those little shining crystals so abundant in some kinds of roofing-slate are cubic pyrites. The name is derived from the Greek, pyr, fire; because the mineral occasionally produces spontaneous combustion. LAMINATION (Lat., lamina, a leaf or blade)—applied to thin layers or leaf-like divisions of rocks. ARENACEOUS (Lat., arena, sand)---sandy. Rocks chiefly composed of in a stalagmitic form, like the calcorecus spar onflown the sırala”, Su N. Boni. Rev. No 28, p. 456 wee p. 118 post OLD RED SANDSTONE SYSTEM. 91 GRITS-hard sandstone, in which the grains of quartz are sharp and angular, are technically called grits, as millstone-grit, grindstone-grit, &c. CLEAVAGE. The peculiarities of this structure have given rise to many speculations and experiments. Mr R. W. Fox submitted a mass of moist clay, worked up with acidulated water, to a weak electric action for several months; and it was found at the end of that time to present traces of cleavage, the laminæ being at right angles to the electric forces. Others are of opinion that cleavage is superinduced, when considerable chemical action takes place in any finely pulverised substance as clay-cleavage being thus regarded as a species of rude crystallisation. Another class of theorists, from observing that slaty cleavage occurs among the shales of the coal measures, when these are in the neighbourhood of igneous rocks, attribute the structure to heat. It is not unlikely that all these causes may have been concerned in producing cleavage; for, when better understood, it is more than probable that heat, electricity, and chemical action, are only modifications of one universal agency. OLD RED SANDSTONE SYSTEM. 170. Until a comparatively recent period, geologists were accustomed to consider gneiss, mica schist, clay-slate, and old red sandstone, as sufficiently distinctive of all the stratified systems which lay underneath the coal measures. We have. seen, however, that, in point of mineral composition, as well as in organic remains, the clay-slate differs essentially from the grauwacke; and that grauwacke, as we ascend, begins to lose its arenaceous character, and to be succeeded by a series of argillaceous and calcareous beds more prolific in fossils, and in the mass perceptibly different. To this series of strata Mr Murchison applied the term Silurian system—a system which may be said to partake of the character of the grauwacke beds beneath, as it insensibly passes into the gray micaceous flagstones of the old red sandstone above. The student must not, however, suppose that all these systems are to be met with fully developed in every country; all that the science of geology affirms is, that, when they are present, the above is their order of occurrence, and the general aspect and character they assume. Sometimes, indeed, the clay-slate is represented by a few indistinct argillaceous beds; and in many places it is difficult to discover anything like a well-defined series of strata corresponding to the silurian rocks of Mr Murchison; but, generally speaking, there is always some shade of distinction, either in mineral or fossil character, which enables us to trace the gradation of these successive systems. Whatever difficulty may be experienced in ascertaining the presence and limits of the grauwacke and silurian systems, there is seldom any doubt as to the old red sandstone, which, in the British islands, is one of the most clearly developed of rock formations. *b.) Profloor Edw. Forbes supposes that the Silopian Seas were. I first very deep, & Ivanquil, from Small size of Conchifera, great numbo Sleaters, absence of fossil fish (2) deep. natir forms of most of the plenty o absence of land plants. The Pentaniras Oblongus separatus Upper & Lower Silurian rocks of Surans). – Silurian deposių formed far land/: Allos, diep water, 171. The composition of the old red sandstone, as indicated by the name, is chiefly arenaceous, presenting a succession of sandstones alternating with subordinate layers of sandy shale. The sandstones pass, in fineness, from close-grained fissile flags to thick beds of conglomerate, the latter being composed of pebbles from the size of a hazel-nut to that of a man's head. The whole system is tinged with the peroxide of iron, the colours ranging from a dark rusty gray to brick-red, and from a mottled purple and fawn shade to a cream-yellow. The mottled aspect is principally found in the shales which, from their sandy character, may be regarded as imperfect sandstones. There are also some calcareous beds in the system, but these are not regularly developed, and are all siliceous and concretionary in their composition and texture. From their impure and concretionary aspect, they are generally known by the name of cornstones, and are of little or no use as limestones. Taken in the mass, the composition of this system is sufficiently indicated by the term old red sandstone-the epithet "old" being applied to distinguish it from another series of red sandstones which occurs above the coal measures, and is usually designated the new red sandstone. 172. The order of succession among the old red sandstone strata varies considerably in different localities. It has been stated that the prevailing mineral characters are-fine-grained red sandstones, including detached pebbles; beds of coarse conglomerate; fine-grained fissile micaceous beds of a gray colour, locally called tilestones and flagstones; layers of mottled shales; and strata of yellow sandstone. Now, although in some districts the conglomerate may be undermost, and in others the tilestones, yet, making allowance for these local deviations, the following may be taken as the most frequent order of occurrence : COAL MEASURES. 1. Yellow sandstones, fine-grained, including detached pebbles, and alternating with layers of mottled shale. Remains of fishes, but no traces of vegetables. 2. Red conglomerate, or Puddingstone, of vast thickness, either in one mass, or interrupted by occasional beds of red sandstone. No organic remains. 3. The red sandstone proper, generally in thick beds of a brickred colour, enclosing detached pebbles of quartz and other primary rocks. Conglomerate beds and concretionary limestones are occasionally interstratified. Organic remains rare, and not very distinct. 4. Gray micaceous beds, sometimes dark and bituminous. These vary in thickness, from one inch to several feet. Remains of fishes abundant; some vegetable impressions. GRAUWACKE. of Agazziz has announced that the genera of shark of The division, Cestracian occur in the Devonian bes The preceding synopsis represents the usual order of the system as it occurs in Scotland. All of these groups are well marked in the field; and when treated not as distinct systems, but as portions of one great system, materially aid the investigations of the geologist. 173. The organic remains of the system, if not so numerous as those of the grauwacke beneath, or the carboniferous measures above, are at least equally interesting, on account of their peculiarities and adaptation to the conditions under which they were destined to exist. The remains of plants are few and indistinct; but are apparently allied to those found in the true silurian rocks. In the tilestone group have been found impressions of ferns, equisetaceæ, leaves resembling those of the flag and flowering rush, and circular markings like the floral envelopes or berries of some shrubby plant. Most of these are highly carbonised and broken, as if drifted from a distance by water, and deposited among the sandy material in which they are now imbedded. A few carbonaceous layers occur among the schistose beds; but vegetable matter nowhere abounds in sufficient quantity to form bituminous layers or thin seams of coal. Taken as a whole, the old red sandstone system is particularly barren of vegetable remains, and seems to evince a condition of the earth which did not permit of the growth of plants unless in detached and limited areas; these plants being by no means high in the scale of vegetable organisation. Its animal remains are more abundant and distinct; but present little variety-the prevailing types being marine fishes of simple but curious structure. 174. The fossil fishes, or ichthyolites, of the old red sandstone, present the first distinct trace of the existence of the highest division of the animal kingdom; namely, vertebrata. It must be remarked, however, that the earliest genera are not of the most perfect structure; but form, as it were, a link between the humbler crustacea and fully-developed fishes. The cephalaspis, coccosteus, and ptericthys, represented in the following engraving, are the most prominent types of these crustacean-like families. The cephalaspis, in general figure, resembles the asaphus of the silurian rocks, is covered with bony plates, and takes its name from the buckler-shape of its head (Gr., kephale, the head, and aspis, a buckler). The coccosteus is also enveloped in a bony covering, is furnished with a tail for locomotion, and takes its name from the berrylike tubercles which dot its plates (Gr., kokkos, a berry, and osteon, a bone). The ptericthys has the same kind of covering or external skeleton; but its distinguishing feature is a pair of wing-like appendages, which seem not only to have aided |