direction of these fissures is such as to indicate a general and long-continued agency pervading the whole strata. In Great Britain, two-thirds of them run north north-west to south south-east, and the remaining third at right angles to that direction, independently of the dip or strike of the strata. Their origin is attributed to contraction during the consolidation of the strata; to expansion and contraction by alternations of Fig. 66. temperature; and to electricity. In some sandstones and beds of ironstone, there are numerous and irregular fractures or seams dividing the surface into

small polygonal areas with a concentric structure,

as exhibited in Figure 66.

111. Some slate rocks are capable of indefinite subdivision in a direction not coinciding with the planes of stratification, nor with joints. This is termed cleavage. The direction of the cleavage planes appears to be, generally, parallel to the anticlinal line of the region in which the rocks occur, and is altogether independent of the dip of the strata. The strata on the two sides of a mountain chain may dip in opposite directions, while the cleavage planes are vertical between them, and parallel to the anticlinal axis, as is the case in the Alps. The phenomena of cleavage in rocks are ascribed to crystallization, or a re-arrangement of the par

ticles of the strata, by which similar materials are collected in planes.

CLASSIFICATION OF ROCKS.

112. While little difference of opinion exists among geologists with reference to the general arrangement of the rocks, several systems of classification have been proposed by them. These systems differ in the grouping of particular strata and formations. Their diversity is a source of distraction to the student of geology, on account of the multiplicity and discordance of the terms, which they introduce. All geologists agree in the division of rocks intostratified and unstratified-fossiliferous and non-fossiliferous-and in the invariable order of succession of the stratified fossiliferous; but it is not practicable at present to determine, in geological formations, the relative places of classes, orders, genera and species, with that accuracy which characterises some other branches of natural science.

113. The fundamental idea involved in systems of classification is the relative age of rocks, and formations. This, in the case of stratified rocks, is determined by the position of the strata; by the characters of the animal and vegetable bodies they contain; and by their mineral constituents.

114. The order of superposition of strata is manifestly indicative of their relative age, since the lowest stratum, upon which the others lie, must have been first deposited, and the others in order upward. But a difficulty attends the investigation of this order of succession, on account of the absence of some of the strata at any one place of observation. There is no place on the globe, where, if a section were made through the rocks, all the strata would be found,

because the strata are not continuous round the earth, like the coats of an onion. Those parts of the globe that constituted the dry land while any deposit was forming in the seas, would receive no portion of the deposit, and as irregularities of distribution of land and water have always existed, parts of the surface must have successively constituted continents and islands. This difficulty in observation is obviated by the meeting and overlapping of the various formations. Thus, if six formations be represented by the first letters of the alphabet a, b, c, d, e, f, in their order, and at one place the formations b and e be missing, observation at another place may give the formations a, b, c, d, f, thus supplying the first deficiency b; and further investigation at other points, present c, d, e, f, thus enabling us to determine the order of succession of the six formations.

115. Since each formation has fossils, remains of animal and vegetable bodies, peculiar to itself, we are enabled by means of them to determine whether strata which are remote from each other geographically, as in America and Europe, were deposited at the same or different periods. A difficulty may seem to arise here, from what has been stated ($87) respecting the different animals and plants which live at present in different localities, but the differences between the faunas and floras of different geological periods are much greater than those which exist between the animals and plants of any one period. The difficulty, in the case of the older formations, is also much diminished by the great uniformity which characterised the faunas and floras of those periods; they were less numerous and more widely extended than at the present period. The relative ages of the more recent formations may be determined to a certain degree, by observing the number of animals and

plants contained in them which are identical with species living at the present time. This number continually diminishes as we recede from the present geological period, until no trace of the species that live at the present day is found. When two geological formations contain many fossils in common, we infer that they were formed at about the same period.

116. Some minerals have been deposited, at certain periods more abundantly than at others; still, different minerals have been deposited at the same period, their distribution depending upon local circumstances, as at the bottom of the present ocean, in some places limestones are forming, at others clay-beds, and at others sandstones. Identity of mineral constitution, therefore, does not prove strata contemporaneous, nor does the failure of that idenity necessarily indicate a different period of origin. When the fragments of rocks of one formation are included in those of another, we have evidence that the rocks to which these fragments belonged were formed, consolidated and fractured before the others were deposited. The conglomerates, or pudding-stones, are filled with worn, rounded fragments of other rocks; indeed all the sedimentary rocks consist of fragments, fine or coarse, of rocks older than themselves.

117. As the unstratified or igneous rocks occur in no regular order of succession, their age is not always easily determined. Their relations, however, to the stratified rocks furnish some intimations of their relative ages. When an igneous rock has passed through a stratum, causing dislocation or changes of structure, it is manifestly more recent than the stratum. A volcanic rock, as lava, may flow over strata producing its characteristic effects upon them, and subsequently other strata may be deposited upon it, accom

modating themselves to its form, but experiencing no heating effects from it; we can, in such case, identify the period of its eruption, as the one which elapsed between the deposition of the two beds.

118. In A. D. 1680, Leibnitz divided all rocks into two classes-stratified and unstratified-in accordance with their origin. Subsequently Lehmann, a German mineralogist, classified the stratified rocks, as 1. Primitive-those which contain no animal or vegetable bodies; 2. Secondary-those which contained plants and animals; and 3. Local-those which occurred in limited localities. Werner