Radiated, when the fibres are broader and flattish and diverging. Lamellar or Foliated, composed of minute plates laid over each

other.

Porous, penetrated by pores.

Cellular, or Vesicular, when the pores swell into rounded cavities, like bladders, as in some lavas.

Slaty, or Laminar, composed of straight, parallel, thin plates, or laminæ.

The structure of compound rocks, may also be Slaty.

The external structure of rocks en masse, or considered as mountain masses, is as distinct from their internal mineral structure, as the shape of a building from that of the bricks or stones of which it is composed; though this distinction has been generally overlooked. The external structure of rocks, as forming mountain masses, may be Stratified, or stratiform.

Tabular, or in large plates.

Columnar,

Globular, or in spherical masses.

Massive, or Indeterminate, which includes all unstratified rocks that have no determinate shape.

Stratified mountains or rocks are those which are composed of layers of stone, laid over each other, and divided by parallel seams like the leaves of a closed book. In these seams or partings, which divide the strata, there are frequently, thin lamine of soft earthy matter; but, sometimes, the surfaces of the upper and lower stratum are so closely joined, that it requires a considerable force to separate them. These layers are denominated strata: they extend through the whole mountain or mass, their length and breadth, being much greater than their thickness. If the thickness of any stratum exceed two or three yards, it is more usually denominated a bed; and if it lie between two beds of stone of a different kind, it is said to be imbedded. Strata, almost always, decline, or dip down to some point of the horizon, and of course rise towards the opposite point. A line drawn through these points is called the line of their dip: another line drawn at right angles to this, marks the course along which the strata stretch out to the greatest extent :-it is called the line of bearing. If a book be raised in an inclined position, with the back resting lengthwise upon the table, the leaves may be supposed to represent different strata; then, a line descending from the upper edges to the table will be the line of dip, and their direction lengthwise will be the line of bearing; and the angle they make with the table will be the angle of inclination. Strata, are, however, sometimes curved or bent in both directions, and are frequently broken; which makes it difficult to ascertain their true position.

Stratified rocks of sandstone, and beds of clay and marl, are generally admitted to have been deposited by the turbid waters of the sea, or of large rivers or lakes. These sedimentary depositions are

arranged over each other in regular layers; the particles or fragments of which they are composed vary in size, and indicate the different states of agitation or repose of the waters from which they were deposited. It is proper to notice, that certain rocks are disposed to divide in parallel seams, in a different direction from that of the regular stratification: this results from the crystalline structure of the rock. Some strata appear to have been formed by chemical precipitation; and, not unfrequently, chemical precipitation and sedimentary deposition have taken place, at the same time, and produced rocks of a mixed character.

The Tabular structure consists of parallel plates 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 occurs chiefly 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 VII.) 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 columnar structure; but the structure is seldom so regular as in basaltic rocks.

CHAPTER IV.

ON STRATIFICATION, AND THE RELATIVE POSITION OF ROCKS.

Strata and Geological Formations explained.-Various Appearances presented by plane 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 Continuity of stratified Rocks broken by Valleys.-Longitudinal Valleys-Transverse Valleys.-Lateral Valleys.-Denudations.-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.-Different Ages of Mountain Ranges.

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, much intermixed, in the lower part, with beds of sand. If we bore through this clay, we shall find its average thickness to be nearly 300 feet. When we have pierced through this, 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 over each other in the same order of superposition. 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 I. fig. 1. Now,

The lower clay is by some geologists denominated plastic clay. See Chap. XIV.

by travelling over 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 position and order of 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 may imagine; and for want of it, geologists 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 minutiae 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 a stratum 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 of rocks of another kind intervening which recur in different parts of the series, they are regarded as having been all formed, nearly at the same epoch, and under similar circumstances; and such series are called, by geologists, Formations. Thus, the strata of shale, sandstone, and ironstone that accompany beds of coal are called the Coal formation. Strata of different kinds, in which a gradation into each other is observed, and which contain similar species of organic remains, also constitute a geological Formation. The chalk with flints, the lower chalk without flints, the chalk-marl and the green sand under the chalk, are regarded as members of what is denominated the Chalk formation. The student, however, must be careful to distinguish the different meaning of a rock formation, as here described, and the formation of a rock: the latter term implies the mode of formation, or the agent by which the rock was formed or consolidated; whether by igneous fusion, as beds of lava; by deposition from water, as beds of clay and sandstone; or by animal secretion, as beds of coral.

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 up,-say the western side, we may suppose the pasteboard plane to represent a stratum, rising to the west and dipping eastward. The lengthwise 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; and 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 be so bent as to dip two ways in the same mountain, like the sloping sides of the roof of a church, or the letter V reversed (A). (See Plate I. fig. 2. stratum 4. and 5.) 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 west on the other. The limestone strata at Dudley Castle Hill dip on each side of the hill as above described. (See Plate III. fig. 4. B.) When strata are bent on each side of a mountain, without being broken at the top, they are called saddle-shaped. A line traced on the surface of a country, to designate where the strata dip in opposite directions, has been called the anticlinal line, and should be introduced in all geological maps, when it can be conveniently ascertained.

Whatever may be the inclination of a stratum, its true thickness is measured by a line perpendicular to the upper and under surface.

If we take a number of similar planes of pasteboard of different colours, and lay the undermost a little inclined, and place another plane upon it, with the upper edge about an inch or more distant from that of the under stratum, and again lay the others in succession in the same manner; the uncovered ends of the planes will rise from under each other, like a number of slices of bread laid on a plate. These uncovered edges will represent the outcrop or crop of the strata, and it will be perceived how we may obtain a knowledge of an under stratum without sinking or boring, merely by crossing a country in the line of the rise or dip of the strata. When strata are arranged in this manner, they are said to be in a conformable position. (Plate I. fig. 1.) It will naturally be enquired whether the strata absolutely terminate where we find their outcrop. In some instances this is the case; but frequently the strata are bent or