« ΠροηγούμενηΣυνέχεια »
marked, if not so easily examined. Indeed, varieties, or to give terms by which they may be without any experiment to ascertain this fact, it discriminated, as they cannot fail to occur to might have been pretty certainly inferred from every one in the least acquainted with the eleanalogy, when we consider that the ocean covers ments of geography. more than a half of the crust of the earth, and The direction and magnitude of rivers frethat the various islands and continents that com- quently indicate the direction in the inequalities pose the exposed part of it, are merely to be of the regions through which they flow, and in viewed as elevations of greater or less extent, their course sometimes present us with particular that rise out of its waters, and continuations of appearances, which have not yet been enumethose below them. The bottom of the ocean rated in any of the foregoing divisions. We may therefore be as much diversified by moun- allude to those basins, or concavities, which they tains and valleys as the surface of our continents. seem once to have formed or occupied, and The most general division of the dry land is into through which they now flow. These concavihigh and low land. The low land is composed ties are of various shapes, sometimes inclining to principally of plains, though it admits sometimes circular or oval, and other times more elongated. of mountainous and hilly ground of no consider- The lips of the basin seem in general to rise able elevation. It does not rise much above the pretty high from its bottom, and to be composed level of the sea, and is made up principally of of rocks or earths in a different state of aggregaalluvial deposits or of secondary forinations. The tion. In the basin itself the surface consists high land is composed of alpine, mountainous, chiefly of gravel, sand, clay, loam, or chalk, that and hilly ground, each rising, above the other, appear to be late mechanical deposits from water, and forining a suite of elevations from the low or rolled and rounded by its action. The sides land to the most prominent points of a continent. or lips of it are either parts of mountains, or siThe general distribution of the high lands we have milar to the ground without it in.structure. Exstated in our preceding article on GEOGRAPHY. amples of these concavities will occur to almost
The characteristic feature of the mountain every one. They are met with in the course of the groups is the superior elevation of its central Rhine, the Rhone, the Elbe, and the Danube. chain, and the gradual sinking of the moun- The Vallais, and the concavity in which the lake tain masses towards its extremities. Mountai- of Geneva is situated, may serve to show this nous land is distinguished from Alpine by its appearance in the line of the Rhone; the lake of less height, and by its want of that aggrega- Constance in that of the Rhine; the circular tion of eminences that constitute the mountain- valley of Bohemia, and the plain on which group; and hilly land, although it sometimes Dresden is situated, iu that of the Elbe; and the possesses this appearance, is distinguished from Danube, beside many others, traverses a basin the two former by its less elevation, and by the of this kind that composes a great part of Hunrounded wary aspect of its inequalities. From gary. They occur in Scotland in Dumfriesshire, the central chain of the mountain-group all the Perthshire, and Aberdeenshire. The Tay at other parts decline on both sides. It is never Perth, before issuing from the narrow pass beintersected totally in its course, and is generally tween the hills of Kinnoul and Moncrieff, points commensurate in length with the extent of the out one instance of them. group.
It is divided from those subordinate These inequalities, which we have enumerated chains that are arranged with it, by valleys fre- under the terms of low land and high land, quently traversed by rivers. The mountain- mountain-groups and alpine summits, are not chain is subdivided into individual mountains, more remurkable for the manner in which they which are separated from each other by small diversify the surface of the earth, than for the valleys or ravines, neither wide, deep, nor long, difference of the mineral masses which they exand seldom permeated by streams and rivers. hibit. The more elevated regions are composed
Most mountains have a foot, an acclivity, and a of bare cliffy rocks, masses generally crystallised summit; meaning by these terms, what is gene- to a certain extent, and in some cases highly inrally understood by them, the lowest portion, the durated, and presenting pointed abrupt surfaces. intervening space, and the highest point. Some- As we descend, the rocks lose their crystalline times the acclivity is perpendicular, and then appearance, and we meet with slime and clayey there can be no foot; sometimes inclined at an fossils in greater abundance. Still lower down, angle, that will not permit the growth of trees; we find fragments of rock, rolled pieces of sandand sometimes possessing such a gentle inclina- stone, sand, soil, or mould, and other soft, distion as allows it to have soft mould, and to be- continuous, and fortuitously arranged substances. come fertile in vegetables. The character of the Another remarkable distinction of the fossils, exsimmit is very various, according to the kind of imined at these different altitudes, consists in rock that predominates, and other causes. cheir state with regard to petrifactions of organic Granite and limestone often present angular, remains. In the higher elevations, and among precipitous, and sharp-pointed summits, as may the fossils highly crystallised, such as granite, we be seen in the Alps, the Appennines, the rock of can discover no trace of the exuviæ of organised Gibraltar, and other mountains. Gneiss forms being, no petrifaction of animal forms. In the flat or round-backed summits; basalt, and some middle regions these appearances are rare, and other, obtuse conical ones.
exhibit little variety. They increase in abunMountain-groups vary very much in their dance, and in the perfection of the animal struclength, their breadth, the shapes of their indivi- ture, as we descend, till we arrive at petrifacdual mountains, and the height of their summits. tions of the animals that now inhabit the earth It is almost needless to mention instances of these along with us. That many of those appearances
which we have pointed out in the inequalities of , by the eye to be connected with a cement, and the earth's surface have been modified by the occurs in brecia, pudding-stone, and other action of the elements both mechanical and che- fossils. The appearance of this structure allows mical, in a long course of ages, cannot be de- us to infer, that the materials of the stone, where nied, and might be admitted with greater faci- it is found, are not at present in the situation lity, did our limits permit us to describe the pro- where they were first formed into a solid ; but cesses that are now advancing, under the same that they have been broken off from larger fragcauses. Differences of temperature in promoting ments, that they have been carried from their a disunion of contiguous portions; great floods original place, and, after being modified in their or continued falls of rain in swelling rivers, course, have been deposited from a fluid, and brooks or mountain torrents, and enabling them connected by a finer extraneous substance. The to detach and carry along pieces of rock or parts aggregated structure is distinguished from the ceof loose soil from the higher to the lower ground; mented, by the immediate connexion of the parts the masses of snow and ice accumulated during of the rock where it occurs, without the interventhe colder scasons, and precipitated on the lower tion of cement. Of the aggregated structure ground by thaws; the powers of violent storms there are two kinds, simple and double, and each and earthquakes on the shores and lower levels, of these has its own subordinate differences. are arnong the agents whose forces and effects Simple aggregation may characterise two states ought in such a view to be appreciated. Nor of a body. One of the component parts may would the operation of chemical agency be un- predominate so much as to constitute a basis in deserving of notice, when we consider the vast which the others are inbedded, and consequently quantities of lime and other matters that are may prevent these others from mutual cortact; held dissolved in springs and streams, and which, or the fossils may be so arranged, that all the being deposited gradually from their waters, in- parts may be brought into immediate and recicrust the edges of their channels, or petrify the procal connexion. The arst of those kinds of wood upon their banks. Appearances of this simple aggregation is aga 'n divided into two kind occur frequently in the Velino, the Teve- subordinate differences, denominated the porrone, and many other rivers of Italy, and pre- phyritic and amygdaloidal structure. The chasent us sometimes with the appearance of petri- racteristic distinction of a porphyritic mineral is fied thickets. The deposition of saline sub- the dispersion of one of its constituent parts stances in many places, and the stalactitical form through a basis in the shape of grains or crystals; in which fossils of the flint genus sometimes the amygdaloidal structure, on the other hand, occur, demonstrate that chemical precipitates presents vesicles or spongy-like spaces, dispersed from water are not confined to calcareous mine- irregularly through a basis, sometimes half-filled, rals. From these facts we may infer, that many sometimes empty, and sometimes completely of the phenomena on the earth's surface, which filled. The crystal or grains in the first kind of must have taken place in remote periods, are rock appear to be of contemporaneous formation similar to those mechanical and chemical results, with the base, the matter that fills up the vesithe causes of which are now in action; and that cular spaces in the amygdaloidal structure. we have thus, as it were, before our eyes, though Those fossils that possess that kind of simple age on a very diminished scale, a specimen of these gregation, in which all the parts come into conmighty processes by which, under Providence, tact without the intervention of a base, are either our globe was brought into a state fitted for granular or slaty. The granular structure is exbecoming our habitation; but this inference we hibited by rocks whose parts are nearly equal shall leave for future discussion, and proceed in all the dimensions of length, breadth, and at present,
thickness, and appear an aggregate of contempoII. To give a very brief description of the in- raneous formations. The slaty structure, on the ternal structure of the earth's crust, and the rela- other hand, though its constituents seem of tive position of its parts. And here fossils may contemporaneous formation, is composed of be considered either, 1. With regard to the struc- parts whose length and breadth are more consiture of the smaller portions or fragments into derable than their thickness. Of the former kind which they may be divided; or, 2. With regard of aggregation, granite and sienite are good exto their distribution, arrangement, and relative amples; of the latter, gneiss and mica-slate may position, as composing greater or smaller por- serve as instances of frequent occurrence. tions of the frame of our globe. When viewed Besides the aggregation called simple, which in regard to their fragments, or hand-specimens, we have just described, there is what is called they are either simple or compound. Simple the compound aggregation, divided into five disfossils may be defined to be those, the whole tinct subordinate kinds; 1. The granular slaty; mass of which is uniform and homogeneous, of 2. The slaty granular; 3. Granular porplıyritic; which every separate division affords the same ma- 4. The slaty porphyritic; 5. The porphyritic terials, and exhibits the same appearances, and of amygdaloidal. "The five kinds of compound agwhich any given portion may be taken as a speci- gregated structure, it will be seen from the name, men or sample of the whole. Of this kind are lime- are merely combinations of the four simple ones stone, serpentine, and varieties of other rocks. formerly mentioned, and do not require, in an
Compound minerals, from the manner in abridged account like the present, to be particuwbich their particles or individual portions are larly described. It may be proper only to rejoined, are denominated either cemented or ag- mark, that, when the two adjectives descriptive gregated. The cemented structure is formed of the compound siructure are applied to a mass, when the individual particles may be perceived the former expresses the appearance it exhibits
in the small or more confined view of it, and the are the most nurnerous are to be considered as latter in the great, or on the more extensive scale. expressive of the general inclination and dirocThus, to take an example, granular slaty de- tion. It sometimes happens, that this general scribes a specimen which is granular in the small position has also its variations ; these must also but slaty in the great, such as gneiss where the be attended to and noted. An acquaintance with quartz and felspar form the granular texture, and the shape of a mountain group, will assist us the mica is disposed in the direction of the slaty very much in such investigations, as it is intistructure. With these terms, and under the mately connected with the general disposition of head of simple and compound minerals, we
the stratification of the masses of which it is might enumerate and describe the different kinds composed. It is also of importance to know of fossil masses that compose the crust of the the fall or declivity of a mountain group, as its earth. But referring to the body of the work, direction and inclination are generally conforınaand the following head, for a description and an ble, particularly in the older formations, with that analysis of them, we proceed to describe the of the super-imposed masses. Sometimes, instructure of mountain-masses, and of their ge- deed, there are exceptions to this rule; but these neral arrangement and relative position, as con are easily explained. stituting the habitable globe. And here we quote It is often very difficult to determine, whether Mr. Jameson's account of those arrangements, as the rock we are examining be stratified or uot, the passage of his work which contains it ex- and, when the seams of stratification are bid, to presses every thing necessary to be observed, in know the direction of the strata. The following language so perspicuous and concise, that it observations will be useful in removing soine of would be useless to extend it, and almost im- the difficulties attending such investigations. 1. possible to abridge it.
Strata are almost always parallel with the slaty
structure of the stone. In certain porphyritic STRUCTURE OF MOUNTAIN-MASSES.
granites, the crystals of felspar appear to lie In the structure of mountain-masses, we have parallel with the strata; the latter character, two principal kinds of structure to describe. however, is by no means so decisive as the forThese are the stratified structure, or that of strata, mer. 2. Strata can only be formed by parallel and the seamed structure, or that of distinct con seams, which have the same direction and extent cretions.
through the mountain-mass.
Where parallel Stratified structure. When a mountain, or rents occur in different directions in the same mountain-mass. composed of one species of rock, species of rock, as in granite, sandstone, limeis divided, by means of parallel seams, into stone, &c., it is evident that they are to be conmasses, whose length and breadth are greater sidered as accidental. 3. The seams of tabular than their thickness, or into what may be deno- distinct concretions, which are often of considerminated tabular masses, which extend through able extent, must not be confounded with strata the whole mountain, it is said to be stratified, seams, because their extent is not so considerable; and the individual masses are termed strata. Of and in each group of concretions the direction is this kind of structure we have instances in different. 4. Where parallel rents have a diffe:granite, limestone, clay-slate, and mica-slate. ent direction from the slaty structure of the But if the mountain or mountain-mass consists of stone, they are certainly accidental. Inattention an alternation of different rocks, as of clay-slate to this circumstance has led several mineralogists and greenstone, or of gneiss and limestone, into error. A striking instance of these rents it is said to be composed of beds. The seams was observed in a quarry of gneiss, the forest that separate the strata, are named strata seams, of Tharand, in Upper Saxony. The gneiss, a: or seams of stratification.
first sight, appeared to be disposed in vertical Strata are in general from four to six feet strata, and as such it was viewed by De Luc; thick in the older formations; but less consider- on a closer examination, however, the apparent, able in the newer. They also vary very much vertical seams proved to be merely accidental in their position. The examination of this ap- parallel rents, perpendicular to the slaty strucpearance of strata is of great importance to the ture of the stone; therefore the strata were horigeogpost and mineralogical geographer. zontal, not vertical. 5. Beds are always parallel
The position of a stratum is determined by with the strata; these, therefore, point out the observing its inclination, dip, and direction. direction of the strata. 6. Although the slaty
The inclination is the angle which the stratum structure points out to us the direction which the forms with the horizon, and is determined by the strata must have, it does not follow, that a rock, quadrant.
having a slaty structure, is stratified. 7. In The dip is the point of the compass towards sandstone, limestone, and rock-salt, regular and which the stratum inclines.
very extensive stripes are sometimes observed, The direction is the angle which the stratum which have been confounded with true seams of makes with the meridian, and is determined by stratification. An attentive examination, howthe compass. It is always at right angles to the ever, always discovers them traversing the real dip:
Von Buch, in his description In making observations of this kind, it is of of Landeck, and geognostical observations made the greatest importance to distinguish the gene- in Italy and Germany, and Friesleben, in his ral direction and inclination from the partial. observations on Thuringia, describe striking inTo effect this, we must take the results of a stances of stripes resembling strata-seams. number of particular observations, and compare Strata vary very considerably in the angle them together; and those similar angles which which they form with the horizon: they occur VOL. X.
from horizontal to vertical, but the general in- those cases where there are seams which are clination is between horizontal and 45°. The parallel in one direction, but intersect each differences are either original, or have been pro- other in another. The most striking example of duced by subsequent changes. In strata coin- it is the columnar. The columns are sometimes posed almost entirely of mechanical depositions, regular, sometimes approach to the globular any deviation from the horizontal position is form, and occur even curvated. They are from generally to be considered as caused by a a few inches to many fathoms long, the length change of original position. We must be careful, being determined by the direction of the seams however, not to confound highly inclined strata in one direction. In the islands of Staffa and of sandstone with accidentally changed strata. Eigg there are admirable examples of this kind This sandstone is in its original situation, and it of structure. These cclumns are sometimes owes this situation to the great portion of che- collected into groups, and such groups are often mically dissolved matter combined with it, and separated from each other by seams, or rather the inclination of the bottom on which it is rents, which render them more distinct. Such deposited. It may be said, that the bottom on groups may be considered as immense distinct which it rests has been forced up while the strata concretions. The columns of such a group were soft, and has thus given them their present often tend towards a centre, some are parallel or situation, but this explanation will not suffice, perpendicular, others are horizontal; and all this as these strata sometimes rest on walls of clay- variety sometimes occurs in the same bill. The slate; sometimes on loose sandstone; sometimes columns are sometimes jointed, so that the conon faces composed of horizontal beds of sandstone. vex extremity of the one column is fitted to the In strata composed of chemical precipitates, all the concave extremity of the other; and these variety of inclination depends on the inequality of columns are usually composed of globular disthe bottom. If the bottom be very much inclined, tinct concretions. These globular concretions so are the strata ; if it be very flat, the superincum- are composed of curved lamellar concretions. bent strata are also flat. It is therefore a fact, The spaces between the different globular consays this author, that all inclined strata, with a cretions are composed of a looser matter than very few exceptions, have been formed so origi- the concretions themselves; and it is by the nally, and do not owe their inclination to a sub- falling out of this less compact substauce that sequent change.
the structure of such columns is first developed. Respecting the formation of strata and beds, No rock shows this kind of structure more dismany opinions have been proposed. Werner's tinctly than basalt ; in it we have all the varieties explanation is satisfactory. He remarks, that of the seamed structure, from the smallest, which strata and beds appear to be particular and in- is the lamellar distinct concretion, to the largest, dividual depositions from a state of solution or which is formed by the grouping of columns. suspension in water.
This kind of structure occurs also in porphyry The stratified structure, as well may be sup- and greenstone. Lava never presents any of the posed, occurs in many different rocks, and, in a varieties of the seamed structure; a negative more extended view, probably in all. Gneiss, character which sufficiently distinguishes it from mica-slate, and clay-slate, are always stratified; greenstone or basalt, with which it has been granite frequently ; sienite sometimes stratified, confounded. sometimes unstratified; porphyry is seldom stra Another kind of seamed structure, which detified; primitive limestone occurs both stratified serves to be described, is the tabular seamed and unstratified; fætz-limestone, sandstone, and structure. It is distinguished from the lamellar chalk, are most distinctly stratified.
by being always straight and much thicker. It When we examine the structure of a moun- is generally from three to nine feet in length, and tain, we must be careful that our observations be rarely thicker than two or three inches. Basalt, not too micrological, otherwise we shall undoubt- in the lower parts of an individual deposition, edly fail in acquiring a distinct conception of it. has often this kind of structure. At first sight it This will appear evident, when we reflect that the is not unlike stratification. It also occurs in geognostic features of nature are almost all on columnar porphyry: the great scale. In no case is this rule to be The last, or third kind of seamed structure, is more strictly followed than in the examination of the large globular, or massive, in which all the the stratified structure.
dimensions are nearly alike. It occurs alone By not attending to this mode of examination, (that is, without any other kind of structure), and geognosts have fallen into numberless errors, is from one to three or more fathoms in diameter. and have frequently given to extensive tracts of The larger balls show lamellar distinct concrecountry a most irregular and confused structure. tions, which, we may observe, are always more Speculators building on these errors have repre- solid the nearer we approach the centre. The sented the whole crust of the globe as an irregu- roundish balls of granite, found dispersed over lar and unseemly mass. It is, indeed, surprising, low countries, have been considered as bowlder that men possessed of any knowledge of the or rolled stones, and many theories have been beautiful harmony that prevails in the structure formed to account for their transportation. The of organic beings, could for a moment believe it granite of the island of Arran presents this kind possible, that the great fabric of the globe itself, of structure. that magnificent display of omnipotence, should be destitute of all regularity in its structure, and
STRUCTURE OF FORMATIONS. be nothing more than a heap of ruins.
By a rock-formation we understand a deterSeamed struc'ure. This structure is formed in minate assemblage of similar or dissimilar rock
masses, which are characterised by external and in so far as it is composed of rock formations of internal relations as an independent whole, that different magnitudes, laid over each other in is, as a unity in the series of rock-formations. certain directions. These masses are either simple or compound. Under the five following heads, Werner comWhen the mass is uniform throughout, as is the prehends every relation respecting the exten* and case with limestone or sandstone, it constitutes relative position of formations in general. 1. what may be denominated a simple formation. The original extent of formations. 2. Their Granite, gneiss, and mica-slate, are also of this present extent and continuity. 3. The position kind. When dissimilar masses occur in a for- and direction of the strata of formations, in remation, as in the case of black-coal and flætz- spect to the fundamental rock. . 4. The directrap, it is said to be a compound formation. tion of the strata themselves, without reference
Similar rocks are often repeated in very to the fundamental rock. 5. The relation of different periods. Each of these individual de- the outgoings of the strata to the exterior of the positions is a particular formation, and the mountain. whole is denominated a series or suite of forma It may be previously remarked, that when tions. Thus there is a limestone, a porphyry, a one formation lies on another, it is said to rest granite suite, &c.
on it, and the rock on which it rests is termed It is a determinate character of certain forma- the fundamental rock; and the plane, which setions, to constitute the principal mass of the parates the fundamental rock from the formation mountain in which they occur: this is the case that covers it, is denominated the plane of sepawith gneiss, clay-slate, porphyry, and others. ration, which is always parallel with the seams of With other formations, on the contrary, it is as the strata. essential à character to occur only in single beds 1. Of the original extent of formations.in the others, and these are said to be embedded. Werner observes, that the greater number of The older porphyry, limestone, and many others, formations have been universally deposited ; are of this kind. When such individual beds and these he dertominates universal formations. occur in different principal formations (that is, A very few, however, are to be considered as are not confined to a single one), as primitive exceptions; and these he terms partial or anolimestone, and primitive trap in gneiss, mica- malous formations. slate, and clay-slate, &c.; when they, as is the Universal formations extend around the whole case with these, form single independent wholes, globe (not, however, without interruption), and which always continue the same, notwithstand- constitute by far the greater part of the mass ing the difference of the rocks in which they are of which its crust is composed. Almost all the embedded; and lastly, when they form members primitive, transition, and Netz formations, are of a series of formations, as is also the case in universal depositions; of these we may mention these instances, they are to be considered as granite, gneiss, porphyry, limestone, and basalt. independent formations. If, on the contrary, they Partial formations occur only here and there, are confined to one rock mass; if they bear no and in single spots, and accompanied with marks of a whole; and lastly, if they are con- appearances that indicate the partiality of their nected with no series, or suite of forinations, they depositions. Thus, at Wehraw, in Lusatia, there are associated with the formation in which they is an excellent example of a partial formation. are embedded,
and it is said that they are subor. It consists of sandstone, limestone, bituminous dinate to it. Roestone in the second sandstone shale, and iron-clay; and these rest on loose formation, and copper-slate in the first flætz- sand. The sandstone resembles in many relimestone, are examples of this kind. Beds spects that found in other sandstone formations; composed of various fossils sometimes occur in yet it does not belong to any of them, as is different rock-masses. These fossils are usually evident from its position, and the rocks with those of which the rock-mass is principally com- which it is associated. Werner conjectures that it posed, irregularly mixed, or are simple stones. may have been formed.by a small and partial flood. Such beds are usually very irregular, do not The examination of these partial appearances extend through the whole rock-mass, and, in is of much importance, not only in extending our general, exhibit apparently great irregularity in knowledge of the variety of formations, but in all their relations. They are not alone capable connecting the history of the earth more nearly of any discrimination, and are referred to the with that of man. rock in which they occur. They have been, but 2. Of the present extent and continuity of rather improperly, denominated foreign beds. formations. The present extent and continuity When single beds are well distinguished by the of formations is very different from what it was kind of stone of which they are composed, and originally. We find them either extended unif their composition shows certain peculiarities interruptedly over great tracts of country, or (as, for example, the determinate presence of they appear isolated, of little extent, and fremetallic fossils, &c.), they are referred to the quently resembling partial formations. In the particular repositories, which we shall afterwards one case they are said to be unbroken, in the consider particularly. Beds of iron-pyrites and other broken. The broken formations occur in magnetic ironstone are of this kind.
small detached masses ; and these have peculiar STRUCTURE OF THE CRUST OF THE GLOBE.
denominations, according to their position and
shape. The last kind of structure we have to describe When detached portions occur on the summits is by far the most extensive and important. It of hills, these are called cops. When portions is the structure of the crust of the globe itself, occur, filling up hollow spaces between moun