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marked, if not so easily examined. Indeed, without any experiment to ascertain this fact, it might have been pretty certainly inferred from analogy, when we consider that the ocean covers more than a half of the crust of the earth, and that the various islands and continents that compose the exposed part of it, are merely to be viewed as elevations of greater or less extent, that rise out of its waters, and continuations of those below them. The bottom of the ocean may therefore be as much diversified by mountains and valleys as the surface of our continents. The most general division of the dry land is into high and low land. The low land is composed principally of plains, though it admits sometimes of mountainous and hilly ground of no considerable elevation. It does not rise much above the level of the sea, and is made up principally of alluvial deposits or of secondary formations. The high land is composed of alpine, mountainous, and hilly ground, each rising above the other, and forming a suite of elevations from the low land to the most prominent points of a continent. The general distribution of the highlands we have stated in our preceding article on Geography. The characteristic feature of the mountain groups is the superior elevation of its central chain, and the gradual sinking of the mountain masses towards its extremities. Mountainous land is distinguished from Alpine by its less height, and by its want of that aggregation of eminences that constitute the mountaingroup; and hilly land, although it sometimes possesses this appearance, is distinguished from the two former by its less elevation, and by the rounded wavy aspect of its inequalities. From the central chain of the mountain-group all the other parts decline on both sides. It is never intersected totally in its course, and is generally commensurate in length with the extent of the group. It is divided from those subordinate chains that are arranged with it, by valleys frequently traversed by rivers. The mountainchain is subdivided into individual mountains, which are separated from each other by small valleys or ravines, neither wide, deep, nor long, and seldom permeated by streams and rivers. Most mountains have a foot, an acclivity, and a summit; meaning by these terms, what is generally understood by them, the lowest portion, the intervening space, and the highest point. Sometimes the acclivity is perpendicular, and then there can be no foot; sometimes inclined at an angle, that will not permit the growth of trees; and sometimes possessing such a gentle inclination as allows it to have soft mould, and to become fertile in vegetables. The character of the summit is very various, according to the kind of rock that predominates, and other causes. Granite and limestone often present angular, precipitous, and sharp-pointed summits, as may be seen in the Alps, the Appennines, the rock of Gibraltar, and other mountains. Gneiss forms flat or round-backed summits; basalt, and some other, obtuse conical ones. Mountain-groups vary very much in their length, their i. the shapes of their individual mountains, and the height of their summits. It is almost needless to mention instances of these
varieties, or to give terms by which they may be discriminated, as they cannot fail to occur to every one in the least acquainted with the elements of geography. The direction and magnitude of rivers frequently indicate the direction in the inequalities of the regions through which they flow, and in their course sometimes present us with particular appearances, which have not yet been enumerated in any of the foregoing divisions. We allude to those basins, or concavities, which they seem once to have formed or occupied, and through which they now flow. These concavities are of various shapes, sometimes inclining to circular or oval, and other times more elongated. The lips of the basin seem in general to rise pretty high from its bottom, and to be composed of rocks or earths in a different state of aggregation. In the basin itself the surface consists chiefly of gravel, sand, clay, loam, or chalk, that appear to be late mechanical deposits from water, or rolled and rounded by its action. The sides or lips of it are either parts of mountains, or similar to the ground without it in-structure. Examples of these concavities will occur to almost every one. They are met with in the course of the Rhine, the Rhone, the Elbe, and the Danube. The Vallais, and the concavity in which the lake of Geneva is situated, may serve to show this appearance in the hne of the Rhone; the lake of Constance in that of the Rhine; the circular valley of Bohemia, and the plain on which Dresden is situated, in that of the Elbe; and the Danube, beside many others, traverses a basin of this kind that composes a great part of Hungary. They occur in Scotland in Dumfriesshire, Perthshire, and Aberdeenshire. The Tay at Perth, before issuing from the narrow pass between the hills of Kinnoul and Moncrieff, points out one instance of them. These inequalities, which we have enumerated under the terms of low land and high land, mountain-groups and alpine summits, are not more remarkable for the manner in which they diversify the surface of the earth, than for the difference of the mineral masses which they exhibit. The more elevated regions are composed of bare cliffy rocks, masses generally crystallised to a certain extent, and in some cases highly indurated, and presenting pointed abrupt surfaces. As we descend, the rocks lose their crystalline appearance, and we meet with slime and clayey fossils in greater abundance. Still lower down, we find fragments of rock, rolled pieces of sandstone, sand, soil, or mould, and other soft, discontinuous, and fortuitously arranged substances. Another remarkable distinction of the fossils, examined at these different altitudes, consists in dheir state with regard to petrifactions of organic remains. In the higher elevations, and among the fossils highly crystallised, such as granite, we can discover no trace of the exuviae of organised being, no petrifaction of animal forms. In the middle regions these appearances are rare, and exhibit little variety. They increase in abundance, and in the perfection of the animal structure, as we descend, till we arrive at petrifactions of the animals that now inhabit the earth along with us. That many of those appearances which we have pointed out in the inequalities of the earth's surface have been modified by the action of the elements both mechanical and chemical, in a long course of ages, cannot be denied, and might be admitted with greater facility, did our limits permit us to describe the processes that are now advancing, under the same causes. Differences of temperature in promoting a disunion of contiguous portions; great floods or continued falls of rain in swelling rivers, brooks or mountain torrents, and enabling them to detach and carry along pieces of rock or parts of loose soil from the higher to the lower ground; the masses of snow .# ice accumulated during the colder seasons, and precipitated on the lower ground by thaws; the powers of violent storms and earthquakes on the shores and lower levels, are among the agents whose forces and effects ought in such a view to be appreciated. Nor would the operation of chemical agency be undeserving of notice, when we consider the vast quantities of lime and other matters that are held dissolved in springs and streams, and which, being deposited gradually from their waters, incrust the edges of their channels, or petrify the wood upon their banks. Appearances of this kind occur frequently in the Velino, the Teverone, and many other rivers of Italy, and present us sometimes with the appearance of petrified thickets. The deposition of saline substances in many places, and the stalactitical form in which fossils of the flint genus sometimes occur, demonstrate that chemical precipitates from water are not confined to calcareous minerals. From these facts we may infer, that many of the phenomena on the earth's surface, which must have taken place in remote periods, are similar to those mechanical and chemical results, the causes of which are now in action; and that we have thus, as it were, before our eyes, though on a very diminished scale, a specimen of these mighty processes by which, under Providence, our globe was brought into a state fitted for becoming our habitation; but this inference we shall leave for future discussion, and proceed at present, I. To give a very brief description of the internal structure of the earth's crust, and the relative position of its parts. And here fossils may be considered either, 1. With regard to the structure of the smaller portions or fragments into which they may be divided; or, 2. With regard to their distribution, arrangement, and relative position, as composing greater or smaller portions of the frame of our globe. When viewed in regard to their fragments, or hand-specimens, they are either simple or compound. Simple fossils may be defined to be those, the whole mass of which is uniform and homogeneous, of which every separate division affords the same ma– terials, and exhibits the same appearances, and of which any given portion may be taken as a specimen or sample of the whole. Of this kind are limestone, serpentine, and varieties of other rocks. Compound minerals, from the manner in which their particles or individual portions are joined, are denominated either cemented or aggregated. The cemented structure is formed when the individual particles may be perceived
by the eye to be connected with a cement, and occurs in brecia, pudding-stone, and other fossils. The appearance of this structure allows us to infer, that the materials of the stone, where it is found, are not at present in the situation where they were first formed into a solid; but that they have been broken off from larger fragments, that they have been carried from their original place, and, after being modified in their course, have been deposited from a fluid, and connected by a finer extraneous substance. The aggregated structure is distinguished from the cemented, by the immediate connexion of the parts of the rock where it occurs, without the intervention of cement. Of the aggregated structure there are two kinds, simple and double, and each of these has its own subordinate differences. Simple aggregation may characterise two states of a body. One of the component parts may predominate so much as to constitute a basis in which the others are imbedded, and consequently may prevent these others from mutual contact; or the fossils may be so arranged, that all the parts may be brought into immediate and reciprocal connexion. The ‘irst of those kinds of simple aggregation is again divided into two subordinate differences, denominated the porphyritic and amygdaloidal structure. The characteristic distinction of a porphyritic mineral is the dispersion of one of its constituent parts through a basis in the shape of grains or crystals; the amygdaloidal structure, on the other hand, presents vesicles or spongy-like spaces, dispersed irregularly through a basis, sometimes half-filled, sometimes empty, and sometimes completely filled. The crystal or grains in the first kind of rock appear to be of contemporaneous formation with the base, the matter that fills up the vesicular spaces in the amygdaloidal structure. Those fossils that possess that kind of simple aggregation, in which all the parts come into contact without the intervention of a base, are either granular or slaty. The granular structure is exhibited by rocks whose parts are nearly equal in all the dimensions of length, breadth, and thickness, and appear an aggregate of contemporaneous formations. The slaty structure, on the other hand, though its constituents seem of contemporaneous formation, is composed of parts whose length and breadth are more considerable than their thickness. Of the former kind of aggregation, granite and sienite are good examples; of the latter, gneiss and mica-slate may serve as instances of frequent occurrence. Besides the aggregation called simple, which we have just described, there is what is called the compound aggregation, divided into five distinct subordinate kinds; 1. The granular slaty; 2. The slaty granular; 3. Granular porphyritic; 4. The slaty porphyritic; 5. The porphyritic amygdaloidal. The five kinds of compound aggregated structure, it will be seen from the name, are merely combinations of the four simple ones formerly mentioned, and do not require, in an abridged account like the present, to be particularly described. It may be proper only to remark, that, when the two adjectives descriptive of the compound structure are applied to a mass, the former expresses the appearance it exhibits in the small or more confined view of it, and the latter in the great, or on the more extensive scale. Thus, to take an example, granular slaty describes a specimen which is granular in the small but slaty in the great, such as gneiss where the quartz and felspar form the granular texture, and the mica is disposed in the direction of the slaty structure. With these terms, and under the head of simple and compound minerals, we might enumerate and describe the different kinds of fossil masses that compose the crust of the earth. But referring to the body of the work, and the following head, for a description and an analysis of them, we proceed to describe the structure of mountain-masses, and of their general arrangement and relative position, as constituting the habitable globe. And here we quote Mr. Jameson's account of those arrangements, as the passage of his work which contains it ex
resses every thing necessary to be observed, in anguage so perspicuous and concise, that it would be useless to extend it, and almost impossible to abridge it.
STRUCTURE of MoUNTAIN-MAsses.
In the structure of mountain-masses, we have two principal kinds of structure to describe. These are the stratified structure, or that of strata, and the seamed structure, or that of distinct concretions. Stratified structure—When a mountain, or mountain-mass. composed of one species of rock, is divided, by means of parallel seams, into masses, whose length and breadth are greater than their thickness, or into what may be denominated tabular masses, which extend through the whole mountain, it is said to be stratified, and the individual masses are termed strata. Of this kind of structure we have instances in granite, limestone, clay-slate, and mica-slate. But if the mountain or mountain-mass consists of an alternation of different rocks, as of clay-slate and greenstone, or of gneiss and limestone, it is said to be composed of beds. The seams that separate the strata, are named strata seams, or seams of stratification. Strata are in general from four to six feet thick in the older formations; but less considerable in the newer. They also vary very much in their position. The examination of this appearance of strata is of great importance to the * and mineralogical geographer. position of a stratum is determined by observing its inclination, dip, and direction. The inclination is the angle which the stratum forms with the horizon, and is determined by the quadrant. The dip is the point of the compass towards which the stratum inclines. The direction is the angle which the stratum makes with the meridian, and is determined by the compass. It is always at right angles to the dip. % making observations of this kind, it is of the greatest importance to distinguish the general direction and inclination from the partial. To effect this, we must take the results of a number of particular observations, and compare them together; and those similar angles which Vol. X.
are the most numerous are to be considered as expressive of the general inclination and direction. It sometimes happens, that this general [. has also its variations; these must also e attended to and noted. An acquaintance with the shape of a mountain group, will assist us very much in such investigations, as it is intimately connected with the general disposition of the stratification of the masses of which it is composed. It is also of importance to know the fall or declivity of a mountain group, as its direction and inclination are generally conformable, particularly in the older formations, with that of the super-imposed masses. Sometimes, indeed, there are exceptions to this rule; but these are easily explained. It is often very difficult to determine, whether the rock we are examining be stratified or not, and, when the seams of stratification are hid, to know the direction of the strata. The following observations will be useful in removing some of the difficulties attending such investigations. 1. Strata are almost always parallel with the slaty structure of the stone. In certain porphyritic granites, the crystals of felspar appear to lie arallel with the strata; the latter character, owever, is by no means so decisive as the former. 2. Strata can only be formed by parallel seams, which have the same direction and extent through the mountain-mass. Where parallel rents occur in different directions in the same species of rock, as in granite, sandstone, limestone, &c., it is evident that they are to be considered as accidental. 3. The seams of tabular distinct concretions, which are often of considerable extent, must not be confounded with strata seams, because their extent is not so considerable; and in each group of concretions the direction is different. 4. Where parallel rents have a different direction from the slaty structure of the stone, they are certainly accidental. Inattention to this circumstance has led several mineralogists into error. A striking instance of these rents was observed in a quarry of gneiss, in the forest of Tharand, in Upper Saxony. The gneiss, a first sight, appeared to be disposed in vertical strata, and as such it was viewed by De Luc; on a closer examination, however, the apparent vertical seams proved to be merely accidental parallel rents, perpendicular to the slaty structure of the stone; therefore the strata were horizontal, not vertical. 5. Beds are always parallel with the strata; these, therefore, point out the direction of the strata. 6. Although the slaty structure points out to us the direction which the strata must have, it does not follow, that a rock, having a slaty structure, is stratified. 7. In sandstone, limestone, and rock-salt, regular and very extensive stripes are sometimes observed, which have been confounded with true seams of stratification. An attentive examination, however, always discovers them traversing the real strata seams. Von Buch, in his description of Landeck, and geognostical observations made in Italy and Germany, and Friesleben, in his observations on Thuringia, describe striking instances of stripes resembling strata-seams. Strata vary very considerably in the angle which they form with the horizon: "g ncCur from horizontal to vertical, but the general inclination is between horizontal and 45°. The differences are either original, or have been produced by subsequent changes. In strata coinposed almost entirely of mechanical depositions, any deviation from the horizontal position is generally to be considered as caused by a change of original position. We must be careful, however, not to confound highly inclined strata of sandstone with accidentally changed strata. This sandstone is in its original situation, and it owes this situation to the great portion of chemically dissolved matter combined with it, and the inclination of the bottom on which it is deposited. It may be said, that the bottom on which it rests has been forced up while the strata were soft, and has thus given them their present situation; but this explanation will not suffice, as these strata sometimes rest on walls of clayslate; sometimes on loose sandstone; sometimes on faces composed of horizontal beds of sandstone. Instrata composed of chemical precipitates, all the variety of inclination depends on the inequality of the bottom. If the bottom be very much inclined, so are the strata; if it be very flat, the superincumbent strata are also flat. It is therefore a fact, says this author, that all inclined strata, with a very few exceptions, have been formed so originally, and do not owe their inclination to a subsequent change. Respecting the formation of strata and beds, many opinions have been proposed. Werner's explanation is satisfactory. He remarks, that strata and beds appear to be particular and individual depositions from a state of solution or suspension in water. he stratified structure, as well may be supposed, occurs in many different rocks, and, in a more extended view, probably in all. Gneiss, mica-slate, and clay-slate, are always stratified; granite frequently; sienite sometimes stratified, sometimes unstratified; porphyry is seldom stratified; primitive limestone occurs both stratified and unstratified; floetz-limestone, sandstone, and chalk, are most distinctly stratified. When we examine the structure of a mountain, we must be careful that our observations be not too micrological, otherwise we shall undoubtedly fail in acquiring a distinct conception of it. This will appear evident, when we reflect that the geognostic features of nature are almost all on the great scale. In no case is this rule to be more strictly followed than in the examination of the stratified structure. By not attending to this mode of examination, geognosts have fallen into numberless errors, and have frequently given to extensive tracts of country a most irregular and confused structure. Speculators building on these errors have represented the whole crust of the globe as an irregular and unseemly mass. It is, indeed, surprising, that men possessed of any knowledge of the beautiful harmony that prevails in the structure of organic beings, could for a moment believe it possible, that the great fabric of the globe itself, that magnificent display of omnipotence, should be destitute of all regularity in its structure, and be nothing more than a heap of ruins. Seamed structure.—This structure is formed in
those cases where there are seams which are parallel in one direction, but intersect each other in another. The most striking example of it is the columnar. The columns are sometimes regular, sometimes approach to the globular form, and occur even curvated. They are from a few inches to many fathoms long, the length being determined by the direction of the seams in one direction. In the islands of Staffa and Eigg there are admirable examples of this kind of structure. These columns are sometimes collected into groups, and such groups are often separated from each other by seams, or rather rents, which render them more distinct. Such groups may be considered as immense distinct concretions. The columns of such a group often tend towards a centre, some are parallel or perpendicular, others are horizontal; and all this variety sometimes occurs in the same hill. The columns are sometimes jointed, so that the convex extremity of the one column is fitted to the concave extremity of the other; and these columns are usually composed of globular distinct concretions. These globular concretions are composed of curved lamellar concretions. The spaces between the different globular concretions are composed of a looser matter than the concretions themselves; and it is by the falling out of this less compact substance that the structure of such columns is first developed. No rock shows this kind of structure more distinctly than basalt; in it we have all the varieties of the seamed structure, from the smallest, which is the lamellar distinct concretion, to the largest, which is formed by the grouping of columns. This kind of structure occurs also in porphyry and greenstone. Lava never presents any of the varieties of the seamed structure; a negative character which sufficiently distinguishes it from greenstone or basalt, with which it has been confounded.
Another kind of seamed structure, which deserves to be described, is the tabular seamed structure. It is distinguished from the lamellar by being always straight and much thicker. It is generally from three to nine feet in length, and rarely thicker than two or three inches. Basalt, in the lower parts of an individual deposition, has often this kind of structure. At first sight it is not unlike stratification. It also occurs in columnar porphyry.
The last, or third kind of seamed structure, is the large globular, or massive, in which all the dimensions are nearly alike. It occurs alone (that is, without any other kind of structure), and is from one to three or more fathoms in diameter. The larger balls show lamellar distinct concretions, which, we may observe, are always more solid the nearer we approach the centre. The roundish balls of granite, found dispersed over low countries, have been considered as bowlder or rolled stones, and many theories have been formed to account for their transportation. The granite of the island of Arran presents this kind of structure.
STRUCTURE of ForMATIons.
By a rock-formation we understand a determinate assemblage of similar or dissimilar rockmasses, which are characterised by external and internal relations as an independent whole, that is, as a unity in the series of rock-formations. These masses are either simple or compound. When the mass is uniform throughout, as is the case with limestone or sandstone, it constitutes what may be denominated a simple formation. Granite, gneiss, and mica-slate, are also of this kind. When dissimilar masses occur in a formation, as in the case of black-coal and floetztrap, it is said to be a compound formation. Similar rocks are often repeated in very different periods. Each of these individual depositions is a particular formation, and the 'whole is denominated a series or suite of formations. Thus there is a limestone, a porphyry, a granite suite, &c. It is a determinate character of certain formations, to constitute the principal mass of the mountain in which they occur: this is the case with gneiss, clay-slate, porphyry, and others. With other formations, on the contrary, it is as essential a character to occur only in single beds in the others, and these are said to be embedded. The older porphyry, limestone, and many others, are of this kind. When such individual beds occur in different principal formations (that is, are not confined to a single one), as primitive limestone, and primitive trap in gneiss, micaslate, and clay-slate, &c.; when they, as is the case with these, form single independent wholes, which always continue the same, notwithstanding the difference of the rocks in which they are embedded; and lastly, when they form members of a series of formations, as is also the case in these instances, they are to be considered as independent formations. If, on the contrary, they are confined to one rock mass; if they bear no marks of a whole; and lastly, if they are connected with no series, or suite of formations, they are associated with the formation in which they are embedded, and it is said that they are subordinate to it. Roestone in the second sandstone formation, and copper-slate in the first floetzlimestone, are examples of this kind. Beds composed of various fossils sometimes occur in different rock-masses. These fossils are usually those of which the rock-mass is principally composed, irregularly mixed, or are simple stones. Such beds are usually very irregular, do not extend through the whole rock-mass, and, in general, exhibit apparently great irregularity in all their relations. They are not alone capable of any discrimination, and are referred to the rock in which they occur. They have been, but rather improperly, denominated foreign beds. When single beds are well distinguished by the kind of stone of which they are composed, and if their composition shows certain peculiarities (as, for example, the determinate presence of metallic fossils, &c.), they are referred to the particular repositories, which we shall afterwards consider particularly. Beds of iron-pyrites and magnetic ironstone are of this kind.
SIRocture of The CRust of The Globe.
. The last kind of structure we have to describe is by far the most extensive and important. It is the structure of the crust of the globe itself,
in so far as it is composed of rock formations of different magnitudes, laid over each other in certain directions. Under the five following heads, Werner comprehends every relation respecting the exten' and relative position of formations in general. 1. The original extent of formations. 2. Their present extent and continuity. 3. The position and direction of the strata of formations, in respect to the fundamental rock. . 4. The direction of the strata themselves, without reference to the fundamental rock. 5. The relation of the outgoings of the strata to the exterior of the mountain. It may be previously remarked, that when one formation lies on another, it is said to rest on it, and the rock on which it rests is termed the fundamental rock; and the plane, which separates the fundamental rock from the formation that covers it, is denominated the plane of separation, which is always parallel with the seams of the strata. 1. Of the original ertent of formations.— Werner observes, that the greater number of formations have been universally deposited; and these he denominates universal formations. A very few, however, are to be considered as exceptions; and these he terms partial or anomalous formations. Universal formations extend around the whole globe (not, however, without interruption), and constitute by far the greater part of the mass of which its crust is composed. Almost all the primitive, transition, and fleetz formations, are universal depositions; of these we may mention granite, gneiss, porphyry, limestone, and basalt. Partial formations occur only here and there, and in single spots, and accompanied with appearances that indicate the partiality of their depositions. Thus, at Wehraw, in Lusatia, there is an excellent example of a partial formation. It consists of sandstone, limestone, bituminous shale, and iron-clay; and these rest on loose sand. The sandstone resembles in many respects that found in other sandstone formations; yet it does not belong to any of them, as is evident from its position, and the rocks with which it is associated. Werner conjectures that it may have been formed by a small and partial flood. The examination of these partial appearances is of much importance, not only in extending our knowledge of the variety of formations, but in connecting the history of the earth more nearly with that of man. 2. Of the sent ertent and continuity of formations.—The present extent and continuity of formations is very different from what it was originally. We find them either extended uninterruptedly over great tracts of country, or they appear isolated, of little extent, and frequently resembling partial formations. In the one case they are said to be unbroken, in the other broken. The broken formations occur in small detached masses; and these have peculiar denominations, according to their position and shape. Wien detached portions occur on the summits of hills, these are called cops. When portions occur, filling up hollow spaces o moun- 2