of rock, separated by regular seams. This structure has often been confounded with stratification: it appears to be the result of crystallization, and is closely allied to the columnar structure.

The Columnar or Prismatic structure is peculiar to certain rocks, but chiefly occurs in the basaltic and volcanic class. Thick beds are divided into columns or prisms, which are most generally pentagonal. They sometimes form vast ranges of natural columns; as at Staffa, the Giants' Causeway in Ireland, and in many volcanic countries. Sometimes the prismatic structure may be observed forming detached groups of columns and prisms, as represented in the group of columns on Cader Idris, Plate 5. A group of basaltic columns of similar form and equally perfect, was observed by the author on the side of the volcanic mountain called Gravenaire in Auvergne, at a small distance from the crater.

The Globular structure consists of globular masses either detached or imbedded in rocks of the same kind; they are frequently composed of concentric layers.

The terms Massive or Indeterminate, may be applied to all unstratified rocks that have no regular divisions. Many of the primary rocks, such as granite, porphyry, and serpentine, occur in masses of enormous thickness, which are broken by irregular fissures in every direction. Thick currents of lava, which have filled up hollows or valleys, are also indeterminate, as might be expected from their mode of formation. Sometimes rocks of granite and porphyry, and also of compact lava, present either a tabular or a columnar structure; but the structure is seldom so regular as in basaltic rocks.

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CHAPTER IV.

ON STRATIFICATION, AND THE RELATIVE POSITION OF ROCKS.

The Principles of Stratification explained.-Various Appearances presented by plain Strata.-Appearances presented by curved Strata, and Errors respecting them.-Distinction between Strata.--Seams and Natural Fissures or Cleavages.On the conformable and unconformable Positions of stratified and unstratified Rocks.-The Intersection of stratified Rocks by Valleys explained.-Longitudinal Valleys.-Transverse Valleys. Lateral Valleys.-On the Elevation of Mountains and Mountain Chains.-On the Direction of Mountain Chains in the new and old Continents.—On vertical Beds in Mountains. On the apparent Devastation in Alpine Districts.-On the Passages in the Alps called Cols; and Observations respecting their Formation.

WHEN we have ascertained what are the most. common or prevailing rocks in a part of any country, and observed that any one stratum or rock which attracts our attention is in that part of the country invariably covered by a peculiar rock or stratum of a different kind, or invariably covers any particular stratum; we hence learn that there is a certain order of superposition, and we naturally feel desirous to know whether the same order is observable in every country where similar rocks occur. Thus in the vale of Thames round London, there is at the depth of a few feet under the surface, a dark-coloured clay called London Clay; and if we bore through this clay, we shall find its average thick

ness to be nearly 300 feet: but when we have pierced through this clay, we invariably come to chalk; and were we to continue to bore in the chalk, after piercing through many hundred feet of that rock, we should come to a stratum of sand or sandstone filled with green particles, and hence called Green Sand. The observer who had confined his researches to this part of the country only, would form a very erroneous conclusion were he to infer, that the outer crust of the globe was invariably composed of London Clay, Chalk, and Green Sand. But wherever similar beds occur together, they lie in the same order of superposition over each other. Thus the London Clay is never found under the Chalk, or the Green Sand.

But it is not always necessary to bore through the upper beds to ascertain this order: for the different strata scarcely ever occur in a flat or horizontal position; they generally rise in a certain direction, and come to the surface, as represented in Plate 1, fig. 1. Now by travelling in the direction of the strata from A to B, we come upon the outer edges 1 2 3, and may trace their order of succession as they rise from under each other. In ravines and the escarpments of mountains, and in the cliffs on the sea coast, we are also enabled to trace the order of position and succession of rocks. But to do this with tolerable correctness, we must have an accurate knowledge of stratification in all its various possible forms. However simple the principles of stratification may at first appear, this knowledge when applied to practice is not of such easy attainment as some, many imagine, and for want of it Geo

logists of considerable eminence have fallen into the most egregious errors. A knowledge of Stratification is indeed of far greater importance to the practical geologist, than an acquaintance with the minutiæ of Mineralogy or Conchology.

Though the word Stratum in its original language and by general acceptation, in speaking of rocks, denotes a bed, it is convenient to restrict the term bed to strata of considerable thickness; for such beds are often subdivided into several distinct minor strata, and we cannot well describe a stratified stratum.

When a series of strata of a similar rock are arranged with occasional strata intervening of rocks of another kind, which recur in different parts of the series, they are regarded as having been formed nearly at the same epoch, and under similar circumstances; and such series are called by geologists Formations. Thus the strata of marle accompanying chalk, with the strata containing nodules or layers of flint, are, together with the whole series of chalk strata, denominated the chalk formation. In order to obtain a distinct idea of stratification in its simplest form, let the young geologist take a piece of pasteboard or thin wood,-say 12 inches square: let him divide it in the middle into two equal planes, each 12 inches in length and 6 in breadth. Place one of these planes flat on a table with the ends facing the north and south; the sides will of course be at right angles, and face the east and west. Now if one of the sides be tilted,-say the western side, we may suppose the pasteboard plane to represent a stratum rising to the west and dipping eastward. The

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lengthways direction of the plane is called the line of bearing; and the declining direction is called the line of dip, which is at right angles to the line of bearing. The angle at which the stratum rises above the horizontal line or level is called the Inclination. Suppose the western edge of the pasteboard plane is raised above the table, forming with it an angle of thirty degrees; then we say the direction of the stratum is north and south, its dip east, its rise of course west, and its angle of inclination thirty degrees. Simple as this appears, geologists of considerable eminence have made the most palpable mistakes in defining stratification. It has been said correctly, that the line of dip being always at right angles to the direction or line of bearing, when the dip is given, the direction is known but when it is further said, that if the direction is given, the line of dip is given also, the assertion is erroneous; for let the above plane of pasteboard be again laid flat upon the table in the same direction, due north and south; but instead of tilting up the western edge, if we tilt up the eastern, we shall then have the same line of bearing as in the first instance, but the dip will be west instead of

east.

It sometimes happens that a stratum without varying its direction may dip two ways in the same mountain, like the sloping sides of the roof of a church, or the letter A reversed. Place the two planes of pasteboard in a north and south direction, and raise them so as to make the upper edges meet; we shall then have the line of bearing north and south as before, and the dip east on one side and