IV. CLASSIFICATION OF THE MATERIALS COMPOSING THE EARTH'S CRUST INTO SYSTEMS, GROUPS, AND SERIES. 37. In order to arrive at a knowledge of the past conditions of the earth, it is necessary to examine not only the mineral character of the different strata, but to ascertain the nature of their imbedded fossils, their order of superposition, and other physical relations. By such an investigation we are enabled to determine their relative ages, to judge whether they were deposited in lakes, in estuaries, or in seas, and to say what kind of plants and animals flourished at the time of their formation. At the present day, the layers of mud, clay, sand, and gravel depositing in tropical estuaries and seas, will contain less or more the remains of plants and animals peculiar to the tropics: the deposits forming in temperate regions will contain, in like manner, the remains of plants and animals belonging to temperate climates; littoral or shore deposits (littus, littoris, the sea-shore) will contain shells differing from those that occur in the muds of the deeper ocean (pelagic); and should a time arrive when these layers are converted into solid strata, the fossilised plants and animals will become a certain index to the conditions of the regions and areas in which they were deposited. As with existing sediments, so with the strata constituting the solid crust: the lowest must have been formed first, the series beneath must be older than that above it; strata abounding in shells, corals, and other marine remains must have been deposited in the sea, while those containing fresh-water plants and animals give evidence of deposition in lakes or in estuaries. In this way we can determine the nature of any formation, and say whether it has been of marine, lacustrine, or estuarine origin. Again, igneous rocks, which displace and break through any set of strata, must be more recent than these strata ; and if another set of strata overlie these igneous rocks, then must they have been deposited in water at a period subsequent to the igneous eruptions. These and similar propositions are so apparent, that the student can have little difficulty in comprehending the principles upon which geologists have proceeded in classifying the rock-formatious of the globe. 38. The principal guides to geological classification are, order of superposition among the strata, their mineral composition, and the nature of their imbedded fossils. The most superficial observer must have noticed the different aspects of the rocks in different districts, and a little closer inspection will enable him to detect that one set lies always beneath another set, and that while certain shells and corals are found in the lower series, the upper series may contain only the remains of terrestrial vegetation. Thus, in sinking a shaft in the neighbourhood of London, we would pass through thick beds of plastic clay, layers of sand, and strata of water-worn flint gravel; at Cambridge we would pass through strata of chalk; in the east of Yorkshire, through strata of fine-grained sandstones, and soft yellowish limestones called oolite; at Newcastle, through strata of shale, coal, and coarsegrained sandstones; in Forfarshire, through strata of red and greyish sandstones, and conglomerates; while on the flanks of the Grampians, we would pass through beds of roofing-slate and hard crystalline schists. minuter inspection of these strata, we would find that one series lay beneath or was older than another On a series; that the chalk, for example, lay beneath the clays of London; that the yellow limestones of York lay beneath the chalk; that the coals of Newcastle were deeper seated than the oolites of York; and the red sandstones of Forfar still deeper than the coal-bearing strata. Further, when we began to examine the fossil contents of these different strata, we should find each set characterised by peculiar plants and animals-some containing marine shells and corals, some the remains of large reptiles and fishes, and others replete with the debris of terrestrial vegetation. By these methods we would soon be enabled to identify the chalk strata of Cambridge with those of Kent, the oolites of York with those of Bath, the coal-measures of Newcastle with those near Glasgow, and the slates of the Scottish Highlands with those of Cumberland and Wales. As with the rocks of Britain, so with those of every country investigated by geologists; and thus they have been enabled to arrive at a pretty accurate classification of the stratified rocks, both in point of time and mineral character. 39. About the beginning of the present century, the stratified rocks were classified as PRIMARY, TRANSITION, SECONDARY, TERTIARY, and RECENT; and though this arrangement has since been superseded by a more minute and perfect classification, the terms are still in daily use by geologists. Thus we hear and read of "secondary and tertiary rocks;" "rocks of the transition period;" and "fossils of the tertiary epoch." The terms, as well as the ideas they convey, are so firmly grafted on the language of geology, that it is necessary at this stage to present a tabular view of this arrangement: FORMATIONS. RECENT.-All superficial accumulations, as sand, gravel, silt, marl, peat-moss, coral-reefs, &c. Contain the remains of existing plants and animals only partially fossilised. TERTIARY.-Local and limited deposits of regular strata occurring above the chalk. Contain the remains of plants and animals not differing widely in character from those now existing. SECONDARY.-Embracing all the strata known as chalk, oolite, lias, coal-measures, mountain limestone, and old red sandstone. Contain fossil plants and animals of species altogether different from those now existing. TRANSITION.-Strata of slaty and silicious sandstones, calcareous shales, and limestones. Contain few or no fossil plants, and the remains of no higher animals than crustacea, shell-fish, and zoophytes. PRIMARY.-All slaty and crystalline strata-as roofing-slate, micaschist, and gneiss, very hard and compact, and totally destitute of organic remains. 40. By a more extensive examination of the strata in different countries, and especially by a more minute investigation of their fossil contents, these formations of the earlier geologists have since been subdivided into systems, groups, and series. This new arrangement has been founded either on mineral or on fossil distinctions-such differences being sufficient to warrant the conclusion that each set of strata was formed during successive epochs, and under different conditions of external nature. The terms formation, system, group, &c., are often loosely employed by geologists; but in the succeeding chapters we shall use the term system to signify any great assemblage of strata that have a number of mineral and fossil characters in common; the term group, to denote any portion of a system marked by a closer resemblance of mineral and fossil character; the term series, to designate any portion of a group which has some very marked character, either mineral or fossil; and so on with other subdivisions of the stratified formations. A system may thus comprehend several groups, a group several series, and a series may have several distinct stages at which some peculiar forms of life appeared in greatest abundance. Proceeding upon this principle, the stratified rocks may be subdivided into the following systems and groups, which derive their names partly from lithological considerations, as Chalk and Old Red Sandstone; partly from geographical, as Permian and Cambrian; and partly from chronological, as Tertiary and Post-Tertiary: I.-POST-TERTIARY or QUARTERNARY SYSTEM, comprising all alluvial III. CHALK or CRETACEOUS SYSTEM, embracing the chalk and greensand groups. Remains of plants and animals chiefly marine, and belonging to species now extinct. IV.-OOLITIC SYSTEM, comprising the wealden strata, the upper and lower oolite, and the lias. Remains of plants and animals (the most remarkable being huge reptilia), belonging to genera now extinct. V.-TRIASSIC SYSTEM, embracing the upper portion-saliferous marls, muschelkalk, and variegated sandstones-of what was formerly termed the "new red sandstone." Remains of plants and animals more closely allied to those of the oolitic system above than to those of the carboniferous. VI. PERMIAN SYSTEM, embracing the lower portion-magnesian limestones and red sandstones-of what was formerly termed the "new red sandstone." Remains of plants and animals very closely allied, and often generically the same as those of the carboniferous strata. VII.-CARBONIFEROUS SYSTEM, embracing the coal-measures, the mountain limestone, and the carboniferous shales. Remains of plants and animals abundant-the distinguishing features being an excess of endogenous vegetation in the coal-measures, and marine shells, fishes, and zoophytes in the mountain limestone. VIII.-OLD RED SANDSTONE or DEVONIAN SYSTEM, embracing the yellow sandstone, red conglomerate, and grey flagstone groups. Remains of fishes, crustacea, shell-fish, &c., by no means rare; but terrestrial plants comparatively few, and imperfectly preserved. IX.-SILURIAN SYSTEM, embracing the upper and lower silurian groups, or the Ludlow, Wenlock, and Llandeilo series. Remains of crustaceans, mollusca, radiata, zoophytes, and other marine invertebrate animals in abundance. X.-CAMBRIAN SYSTEM, embracing the upper and lower groups, and consisting of hard sub-crystalline slates and grits. Remains of crustacea, mollusca, and zoophytes, with worm-burrows and other marine exuvia. XI.-LAURENTIAN SYSTEM, consisting of crystalline schists, quartzites, and serpentinous limestones. Remains of foraminiferal and other lowly and obscure organisms. XII.-METAMORPHIC ROCKS, embracing the clay-slate, mica-schist, and gneiss groups. All hard and crystalline rocks devoid of fossils. 41. Such are the stratified rocks when arranged in systems and groups; and, so far as geologists have been enabled to discover, there is no deviation from this order of succession. It must not be supposed, however, that all these groups are found at any part of the crust, lying one above another like the coats of an onion; on the contrary, only one or two of the groups may be developed, and these very scantily, and not in immediate succession. All that is meant by order of succession among the stratified rocks is, that wherever two or more systems come together, they are never found out of place; that is, the chalk is never found beneath the oolite, oolite beneath the coal, or coal beneath the old red sandstone. In Fifeshire, for example, the carboniferous system immediately overlies the old red sandstone; in Durham, the new red sandstone overlies the coal; in Yorkshire, the oolite overlies the new red sandstone, and the chalk the oolite; in Kent, the tertiary strata overlie the chalk; and |