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tains, they are denominated upfillings. And when a portion occurs only on one side of a mountain, it is said to be shield-formed, or to hare the shape of a shield.
3. Of the position and direction of strata in regard to the fundamental rock.—When strata have the same direction as the fundamental rock, they are said to be conformable with it; if the direction is different, they are said to be unconformable. If they differ only in direction, we say that they are simply unconformable; but if they differ not only in direction, but lie over the ends of the strata of the fundamental rock, they are said to be unconformable and overlying. Overlying strata occur more frequently than simply uncomformable, and have far greater extent.
4. Of the direction of the strata themselves, ■without reference to the fundamental rock.— Strata are either straight, that is, disposed in one direction on the fundamental rock, or they turn around it, and enclose it; in this case they are said to be mantle-formed, or mantle-shaped. If the strata are not only wrapped around the fundamental rock, but also cover its extremities, they are said to be saddle-shaped.
When the upper part of the saddle-shape is carried away, the mantle-shape is formed.
Strata are sometimes concave, and they are then said to be basin-snaped; but, if the concavity be oblong, they are named trough-shaped. In the concave, that is, the basin and troughshaped, and convex, that, is, the saddle and mantle-shaped strata, the outgoings form circles; in the concave the outer and greatest circle is the oldest; in the convex, on the contrary, the outer and largest circle is the newest
5. Of the relation of the outgoings of the strata to the exterior of the mountain.—We have to consider, first, the relation of the outgoings of strata to mountain-masses of considerable extent; and, secondly, to mountain caps.
In mountain-masses, the strata either cover each other completely, or the outgoings are open and exposed. When the outgoings are exposed, the newer strata have a rising or sinking level.
There are three different kinds of mountaincaps. In the first, the cap rests on a fundamental rock; the seams of the strata are parallel with the plane on which the strata rest, and these are unconformable and overlying; the second kind of cap is formed by a rock rising through the surrounding strata; and the third kind of cap is formed by portions of harder beds remaining after the superincumbent and softer strata have been carried away.
In the first, the rock is unconformable and overlying. The fla>tz-trap and porphyry formations afford numerous examples of this kind of mountain-cap.
In the second, the newer strata are mantleshaped. Granite often occurs in caps of this kind.
In the third, all the strata are conformable, so that the subjacent and superincumbent strata have the same direction with the bed which forms the cap. Primitive greenstone, in clay-slate, sometimes forms caps of this kind.
These strata and formations, and the kinds of
rocks that compose them, follow one another in rising from the lower levels toward the higher; or, in other words, are placed above one another, not in an irregular or promiscuous manner; they follow a certain order of succession, and exhibit certain determinate relations. We do not, for instance, find a stratum of granite alternating with a stratum of sandstone, nor a formation of gneiss covering a bed of pit-coal. In examining the crust of the earth to discover this order of supraposition we observe a great class of rocks pushing in alpine chains, their summits above all the flatter strata, and affording on their sides, and at their base below, a resting place, or, support for them. These rocks are found under every other stratified mass, and never covering, or resting upon, any.
In like manner the rest of the strata have certain relations to each other, from the granite to the most superficial mould.
III. We proceed now to the third division of the subject of geology, which was to give a short view of the various classes of rocks and mineral masses, that compose .the earth's crust, arranged according to their relative situation. We shall best execute this part of our duty, by presenting to our readers Dr. Thomson's compressed, but comprehensive account of the Wernerian arrangement, drawn up with his usual acuteness, and displaying his usual extent of information and research. Some of the observations may be found in the article Mineralogy, and under the titles of the different minerals to which they refer; but we could not on that account omit the repetition of them, without destroying the continuity and systematic connexion of this article.
'Werner has chosen the relative situation of rocks as the basis of his classification. He divides them into five classes. The first class consists of those rocks, which, if we were to suppose each layer to be extended over the whole earth, would in that case lie lowest, or nearest the centre of all the rocks which we know, and be covered by all the other rocks. The second class consists of those rocks which in that case would be immediately above the first class, and cover them. The third class would cover the second in the same manner; the fourth the third; and the fifth would be uppermost of all, and constitute the immediate surface of the earth. The first class of rocks are covered by all the rest, but never themselves lie over any other. The others lie in order over each other. These grand classes of rocks he has denominated formations, and distinguished them by the following specific names: 1. Primitive formations; 2. Transition formation!; 3. Ftaetz formations; 4. Alluvial formations; 5. Volcanic.
The primitive formations are of course the lowest of all, and the alluvial constitute the very surface of the earth; for the volcanic, as is obvious, are confined to particular points. Not that the primitive are always at a great depth under the surface, very often they are at the surface, or even constitute mountains. In such cases, the other classes of formations are wanting altogether. In like manner the transition, and other formations, may each in its turn occupy the surface, or constitute the mass of a mountain. In such cases, all the subsequent formations which ought to cover them are wanting in that particular spot. Each of these grand classes of formations consists of a greater or smaller number of rocks, which occupy a determinate position with respect to each other, and which, like the great formations themselves, may often be wanting in particular places. Let us take a view of the rocks which compose all these different formations.
Class I.-PRIMITive ForMATIONs.
The rocks which constitute the primitive formations are very numerous. They have been divided therefore into seven sets; which constitute as many primitive formations, and are distinguished each by the name of that particular rock which constitutes the greatest proportion of the formation. These seven sets of primitive formations are the following:—1. Granite; 2. Gneiss; 3. Mica-slate; 4. Clay-slate; 5. Newest primitive porphyry; 6. Sienite ; 7. Newer serpentine.
The granite is the undermost, and the sienite the uppermost of the primitive formations. Granite is scarcely mixed with any other rock; but in gneiss, mica-slate, and clay-slate, there occur
of old porphyry, primitive trap, primitive
limestone, old serpentine, quartz rock. For that reason, these rocks are said to constitute formations subordinate to gneiss, mica-slate, and clayslate. Gypsum occurs in beds in mica-slate, and old flint-slate occurs in the same way in clay-slate. Thus, besides the seven principal primitive formations, there occur seven subordinate formations, interspersed through the second, third, and fourth formations; and topaz rock, which lies over gneiss and under clay-slate, must
be added to the list; so that the primitive formations altogether amount to fifteen.
If we suppose the nucleus of the earth to have been first formed, and the formations to have been afterwards deposited in succession upon this nucleus, it will follow that the lowest formation is the oldest, and that the formations are newer and newer according as they approach the surface. This supposition accounts for some of the names given to the primitive formations. That porphyry, for example, is considered as the oldest which lies lowest down in the series of formations, and those formations of porphyry which lie nearer the surface are considered as newer. Granite, of course, according to this way of speaking, is the oldest formation of all, while the alluvial are the newest of all. The following table exhibits a synoptical view of the primitive formations:—
13. Newer porphyry. ". 14. Sienite. 15. Newer serpentine. Let us consider each of them in the order of the formations. 1. Granite is the lowest of all the formations, and the basis upon which the others rest. It is composed of felspar, quartz, and mica; each in a crystallised state, and cohering together without any cement. The felspar is usually the most abundant ingredient, and the mica the smallest in quantity. The color of the quartz and mica is usually gray; but the felspar has a considerable variety of colors, occurring in different shades of white, gray, red, and green. The size of the constituents of granite varies considerably. Sometimes the grains are very large, and sometimes they are so small that the granite has the appearance of a sandstone. Sometimes it is porphyritic, large crystals of felspar occurring in a basis of fine-grained granite. §. this rock is distinctly stratified, but in other cases no stratification can be perceived. The unstratified or massive granite is frequently composed of large globular masses, each of which is composed of concentric lamellar distinct concretions. The intervals between these halls consist of a softer granite, subject to crumble down when exposed to the action of the weather. Besides the three constituents of which granite essentially consists, other crystallised minerals occasionally occur in it, though only in small quantities. These crystals are chiefly of schorl; sometimes garnet and tin-stone. Granite very seldom contains among its strata beds of any foreign rock. Beds of felspar alone have occasionally been observed in it. It is not so rich in ores as some of the other formations. Tin and iron are the metals which are most abundant in it. Hitherto molybdena has been found chiefly in granite. It contains also silver, copper, lead, bismuth, arsenic, cobalt, tungsten, and titanium. Besides the great granite formation, which has been just described, Werner has discovered a second; which is supposed to occur nearly in the same geognostic situation as porphyry and sienite. The granite veins which traverse gneiss, mica-slate, and clay-slate, belong to this formation. There are several particularities by which this newer granite may be distinguished from the other. It usually occurs in a lower level; it has commonly a deep red color; contains garnets; and is not porphyritic. When granite is not covered by any other formation, it forms high insulated cliffs and steep rugged rocks. 2. Gneiss is the formation which lies immediately over granite, and into which indeed it gradually passes. Gneiss consists of the same constituents as granite; namely, felspar, quartz, and mica; but it differs in its structure, being disposed into slates from the prevalence of the mica. The texture of the individual slates is granular. Hence the structure of gneiss is granular slaty. Gneiss is always distinctly stratified. It sometimes contains crystals of schorl; but they are smaller and much more uncommon than in granite. Tourmaline is more common,
and so likewise is garnet. It contains in it many foreign beds, which, is not the case with granite. Beds of three of the first six subordinate formations are found in it. It is, perhaps, richer in metallic ores than any other formation. Almost every metal occurs in it, either in beds or veins.
When gneiss is not covered by any other formation, it forms round-backed mountains, and likewise crags: but less steep and insulated than those composed of granite.
3. Mica-slate.—The formation which lies immediately over gneiss, and into which it insensibly passes, is mica-slate. This rock, like the preceding, is compound, and composed essentially of quartz and mica. Like gneiss it is slaty, but it differs from that rock in containing no felspar. It is always stratified. It very frequently contains garnet crystals in considerable quantity, so as to give it a porphyritic appearance. Its structure is then slaty porphyritic. It sometimes also contains crystals of tourmaline, cyanite, and granatite. Felspar likewise occurs in it occasionally; not, however, as a constituent, but in kidney-form and irregular masses. Like gneiss, it contains many foreign beds (older porphyry, primitive trap, primitive limestone, older serpentine, and gypsum, occur in it). It is rich in ores; containing beds of magnetic ironstone, pyrites, galena, copper pyrites (containing gold), blende, cinnabar, cobalt glance, magnetic pyrites, and sometimes even native gold. It abounds also in metalliferous veins.
4. Clay-slate.—Mica-slate gradually passes into clay-slate, the formation immediately over it. Clay-slate consists essentially of the mineral described in the preceding chapter under that name. This rock is always slaty and always stratified. When it approaches mica-slate, grains of quartz, and also of mica, may be distinguished in it. Occasionally also it contains crystals of felspar, schorl, tourmaline, garnet, and hornblende.
Under the name of clay-slate formation are included not only clay-slate, strictly so called, but likewise chlorite-slate, talc-slate, whet-slate, drawing-slate, and alum-slate; all of which occur along with pure clay-slate, are similarly stratified, r.nd gradually pass into it and into each other, and therefore are considered as only constituting a part of the same formation: but these substances affect a particular order. The following table exhibits that order, beginning with the lowest or oldest, as it is called, and terminating with the uppermost or newest:—
1. Light yellowish gray clay-slate.
2. Dark gray clay-slate.
3. Green clay-slate.
4. Chlorite slate and potstone.
5. Talc slate.
6. Whet slate.
7. Bluish gray clay-slate.
8. Red clay-slate.
9. Drawing slate. 10. Alum slate.
Besides these different beds, which are considered as constituting the clay-slate formation, it contains also beds of all the eight subordi
nate formations. It contains also a considerable number of metallic ores in beds; as iron pyrites, copper pyrites, arsenic pyrites, cpbul t, galena, &c. It contains also a variety of mineral veins.
Having now described the first four primitive formations, let us proceed to the subordinate formations; taking topaz rock along with them, on account of its rarity. These are the older porphyry, primitive trap, primitive limestone, older serpentine, quartz, topaz rock, gypsum, and primitive flint-slate.
5. Older porphyry.—By porphyry, as defined by Werner, is to be understood a rock consisting of a basis or ground of some compact mineral, and in this ground are interspersed crystals of some other mineral. The ground or basis varies, in different porphyries. Sometimes it is claystone, sometimes pitch-stone, &c.; and the porphyry is named from this basis. The following are the species of porphyry that have been described:—1. Clay porphyry; 2. Horn-stone porphyry; 3. Felspar porphyry; 4. Pitch-stone porphyry; 5. Sienite porphyry; 6. Obsidian porphyry; 7. Pearl-stone porphyry.
The crystals interspersed through the different bases are commonly felspar, sometimes quartz, and sometimes hornblende and mica; but the last two are uncommon, especially the mica.
There are two very different formations of porphyry; the first is found in beds in gneiss, micaslate, and clay-slate; whereas the second always lies over all these formations. Hence the first is distinguished by the name of older, and the second is called newer porphyry. It is the first of these that we are to consider at present.
The basis of the older porphyry is usually a species of horn-stone, and sometimes felspar; and the crystals which occur in it are felspar and quartz. Hence the older porphyry consists chiefly of horn-stone porphyry, and felspar porphyry. When not covered by other formations it sometimes forms single rocks, but never large mountains.
6. Primitive trap.—The word trap is Swedish, and signifies a stair. It was applied by the Swedish mineralogists to certain rocks, whose strata when exposed, from the one jutting out under the other, gave an appearance somewhat like a stair. The term was adopted by other nations, and was applied indiscriminately to a great variety of rocks, which bore a certain resemblance to each other. This generalisation introduced much confusion into the subject, which was first cleared up by Werner and his disciples. Under the term traps Werner comprehends certain series of rocks, distinguished chiefly by the hornblende, which they all contain. In the most ancient, the hornblende is almost pure; this purity gradually diminishes, and in the most recent traps the hornblende degenerates to a kind of indurated clay. There are, then, three formations of trap: 1. Primitive trap; 2. Transition trap; 3. Fhrtz trap. The first only occupies our attention at present.
The primitive trap formation contains a considerable number of rocks; which, occurring in different parts of the earth in similar situations, and as it were substituted for each other, are considered altogether as constituting only one formation. The following table exhibits a list of the rocks belonging to this formation:
I. Hornblende. 1. Granular or common hornblende. 2. Hornblende slate.
II. Hornblende and felspar united.
1. Common green-stone.
2. Porphyritic green-stone.
3. Green-stone porphyry.
4. Green porphyry.
- ii. Slaty.
III. Hornblende and mica united. 1. Porphyritic trap.
The first two of these rocks consist essentially ot the minerals described under the names of common hornblende and hornblende slate. The latter sometimes passes into fine slaty gneiss and into chlorite slate. Common green-stone is composed of hornblende and felspar, both in the state of grains or small crystals, and the hornblende usually constitutes by far the greatest proportion of the mass. The felspar is almost always tinged green from the hornblende. This rock sometimes contains a little mica; sometimes it is intersected by small veins of quartz and actinolite; and sometimes also, though less frequently, of felspar and calcareous spar. Porphyritic green-stone, like the preceding rock, is a compound of granular hornblende and felspar; but it contains likewise large crystals of felspar and quartz, interspersed through the greenstone ground. Green-stone porphyry (black P. of the antiquary) is a rock consisting of granular greenstone, so small grained that at first sight it has the appearance of being a simple stone; containing in it large crystals of felspar, colored green from hornblende. Green porphyry (the verde antico serpentine) is a rock which has for its ground a mixture of hornblende and felspar, so intimate that the two ingredients cannot be distinguished by the naked eye, and having a blackish green or pistachio green color. This trap contains greenish colored feispar crystals, often cruciform. Green-stone slate is a rock composed of hornblende and felspar, and sometimes a little mica, and having a slaty texture. It is very hard, but, like the other species of greenstone, soon withers when exposed to the air. Porphyritic trap is a rock composed of an intimate mixture of hornblende and felspar, containing in it large plates of mica, which give it a porphyritic appearance. All the rocks of the primitive trap formation are characterised by containing a mixture of iron pyrites. Their stratification is indistinct; and, indeed, if we except green-stone slate and hornblende slate, usually not perceptible. When primitive trap is not covered by any other formation, it constitutes considerable hills and closis. It abounds in ores, especially green-stone slate.
7. Primitive limestone.—Limestone occurs in all the four grand classes of formations, but it assumes a peculiar and characteristic appearance in each. In the primitive it is distinctly crystalline and transparent. The crystalline texture gradually becomes less and less distinct, as the formations advance, till at last the limestone assumes the appearance of an earthy deposite. Primitive limestone occurs usually in beds, and seldom forms entire mountains. Its color is usually white, sometimes it is gray, but very seldom assumes any other color. It is the species of linestone described already under the name of granular foliated. It sometimes contains in it quartz, mica, hornblende, actilonite, garnet, tremolite, talc, clay-slate, serpentine, asbestus, blende, galena, common and magnetic pyrites, and magnetic ironstone. The ores are usually found at the lower part of the beds of limestone. It is sometimes stratified, and sometimes not. When not covered by any other formation, it forms steep bare rocks; and, when it occurs in considerable quantities, often contains caverns. 8. Older ntine.—The serpentine formation consists essentially of the mineral described already under that name. Two subspecies were thus described; namely, the common and precious serpentine. The latter (at least chiefly) constitutes the formation at present to be described. It occurs, like the other subordinate primitive formations, in beds in gneiss, mica, and clayslate, and alternates with limestone. It is seldom stratified. It contains in it galena, and auriferous arsenic pyrites. 9. Quartz occurs in beds, and in no great quantity. It is usually granular and of a white color; sometimes it contains a mixture of mica, which gives it a slaty texture. It is very often unstratified; but, as it abounds in rents, these have often been mistaken for strata. 10. Topaz rock-The rock which constitutes this formation is very rare, having been observed only in Saxony, where it constitutes a mountain. There it rests on gneiss, and is covered by clayslate. It is composed of three ingredients; namely, fine granular quartz, schorl in thin prismatic distinct concretions, and topaz almost massive. These are arranged in thin layers, and these again are divided into roundish and granular distinct concretions, so that the rock is slaty granular. The intervals between these concretions are filled up with a yellowish or greenish colored lithomarge. 11. Gypsum was formerly believed to be o to the floetz formations; but an immense ed of it has lately been discovered in Switzerland in mica-slate. This primitive gypsum is characterised by containing mica and clayslate. 12. Primitive flint slate.—This rock consists essentially of the mineral, described already under the name of flint slate. It is often traversed by veins of quartz. There are two formations of it; namely, the primitive, which occurs in beds in clay-slate, and another which belongs to the transition formations. Having now described the eight formation,
which are subordinate to gneiss, mica, and clayslates, let us proceed to the remaining primitive formations, which always cover clay-slate, and are therefore considered as newer than i'. These are the newer primitive porphyry, sienite, and the newer serpentine.
13. Newer Porphyry.—To this formation belong the following species of porphyry; namely, clay porphyry, pitchstone porphyry, obsidian porphyry, pearlstone porphyry, and sometimes felspar porphyry. Clay porphyry is by far the most common. To this formation also belongs claystone, a mineral already described. It constitutes the basis of clay porphyry, and frequently occurs without any crystals of felspar or quartz. To it we must also refer a-kind of breccia porphyry, composed chiefly of fragments of hornstone porphyry and felspar porphyry.
14. Sienite.—This rock occurs usually along with porphyry; and, when they are both together, the sienite generally forms the uppermost part of the hill.
Sienite is a rock composed essentially of felspar and hornblende, the same constituents which form green-stone; but in sienite the felspar i-' the prevailing ingredient, whereas in green-stone it is the hornblende. In sienite the felspar is usually red, and very seldom has a tinge of green, whereas in green-stone the felspar is never red, but almost always greenish white. The structure of sienite is granular, and the grains vary greatly in size; sometimes small grained sienite contains in it large crystals of felspar; it is then called porphyritic sienite. When the two ingredients that constitute sienite are so small, and so intimately mixed, that they cannot be distinguished by the naked eye, and when such a rock contains Crystals of felspar and quartz, it is denominated sienite porphyry.
Sienite, like porphyry, contains few foreign beds: but it is rich in ores, containing gold, silver, iron, tin, copper, lead, &c, always however in veins.
The rocks of sienite are often divided into columns. Like the newer porphyry it often occurs in round masses.
15. Newer serpentine.—This formation consists of the rock composed essentially of the mineral called common serpentine. It bears a striking resemblance to the newer porphyry formation, and occurs in similar situations.
Class II.—Transition Formations.
Having described the primitive formations, let us now proceed to the second great class, the transition, which lie immediately over them. These are by no means so numerous, since they consist only of foursets; namely, 1. Greywacke; 2. Transition limestone; 3. Transition trap; 4. Transition flint-slate. They all alternate with each other.
It is in the transition rocks that petrifactions first make their appearance; and it deservts particular attention that they always consist of species of corals and zoophytes, which do not at present exist, and which therefore we must suppose extinct. The name transition has been imposed, as they are supposed to have
been formed when the earth was passing from an uninhabited to an inhabited state.
1. Greywacke.—This formation consists of two different rocks, which usually alternate with each other, and pass into each other. These are greywacke and greywacke slate. The first characterises the formation. Greywacke is a rock composed of pieces of quartz, flint slate, felspar, and clay-slate, cemented together .by a basis of clay-slate. The pieces are sometimes as large as a hen's egg; sometimes so small that they cannot be perceived by the naked eye. The clay-slate basis likewise varies in quantity considerably. It often contains soft plates of mica, and quartz veins are very common in it; sometimes it is so hard that it appears indurated by some siliceous cement. The texture of greywacke becomes gradually finer and finer grained, till at last it can no longer be perceived, and a slaty structure succeeds. It then passes into greywacke slate.
Greywacke slate is in fact a variety of clayslate; distinguished from primitive slate by seldom showing a greenish or light yellowish grey color, or the silvery uninterrupted lustre of primitive slate. It contains no beds of quartz, but very often veins of that mineral; no crystals of felspar, schorl, tourmaline, garnet, or hornblende; nor beds of garnet, chlorite slate, talc, or magnetic ironstone. It contains petrifaction The greywacke rocks are stratified. It contains immense beds of transition limestone, trap, and flint slate; and is rich in ores both in beds and veins, and the veins are often of an uncommonly large size.
2. Transition limestone.—This, like primitive limestone, is a simple rock; but it approaches more to compact, and is less transparent than the primitive. It contains often veins of calcareous spar, and exhibits a variety of colors, which give it a marbled appearance. It contains marine petrifactions of corals and zoophytes, which no longer exist, and which do not occur in the subsequent limestone formations. These petrifactions increase in quantity as the beds of transition limestone advance farther and farther in their position from the primitive formations. It is often mixed with greywacke slate, which gives it a slaty texture. It occurs in beds of greater and smaller size, and often forms whole mountains. It contains no foreign beds except of transition trap. It is scarcely stratified. It is rich in ores.
3. Transition traps.—This formation, as the name imports, consists of rocks distinguished by the great proportion of hornblende which they contain. It comprehends four species of rocks; namely, 1. Transition greenstone. 2. Amygdaloid. 3. Porphyritic transition trap. 4. Globular trap (kugel fels).
Transition greenstone is a rock composed of fine granular hornblende and felspar intimately mixed together, and not so distinctly crystallised as in primitive greenstone. Sometimes the mixture is so intimate that the two minerals cannot be distinguished. In that state it often becomeJ loose in its texture, and approaches waeke and basalt. In this last case it often contains vesi