Part 4

Clinkstone or phonolite is a greenish-gray, compact, felspathic rock, somewhat slaty or schistose, and weathers with a white crust. It gives a clear metallic sound under the hammer. It is a rock not met with among the older formations of the earth's crust, being confined to Tertiary (see table, p. 85) or still more recent times.

Obsidian or volcanic glass is usually black, brown, or green, and usually resembles a coarse bottle-glass. When it becomes vesicular, it passes gradually into the highly porous rock called pumice. It is eminently a geologically modern volcanic rock.

Felstone is a reddish-gray, bluish, greenish, or yellowish, hard, compact, flinty-looking rock, composed of potash-felspar and silica. It is generally splintery under the hammer. Some varieties are slaty, and are frequently mistaken for clinkstone, which they closely resemble. When the quartz in felstone is distinctly visible either as grains or crystals, the rock passes into a quartz-porphyry.

Granite is recognised as an igneous as well as a metamorphic rock. Sometimes the veins and dykes which proceed from or occur near a mass of granite contain no mica--this kind of rock is called elvan or elvanite.

Porphyrite or felspathite includes a number of rocks which have a felspathic base, through which felspar crystals are scattered more or less abundantly. Sometimes hornblende, or augite, or mica is present. The colour is usually dark--some shade of blue, green, red, puce, purple, or brown--and the texture varies from compact and finely crystalline up to coarsely crystalline. Porphyrites are usually porphyritic, and frequently amygdaloidal.

AUGITIC AND HORNBLENDIC OR BASIC GROUP.

32. Basalt is a dark or almost black compact homogeneous rock, composed of felspar and augite with magnetic iron. An olive-green mineral called olivine is very frequently present. The coarser-grained basalts are called dolerite. The columnar structure is not peculiarly characteristic of basalt. Many basalts are not columnar, and not a few columnar rocks are not basalts.

Greenstone or diorite is usually a dull greenish rock, sometimes gray, however, speckled with green. It is composed of soda-felspar and hornblende. The fine-grained compact greenstones are called aphanite.

Syenite, like granite, is recognised as an igneous as well as a metamorphic rock. There are several other rocks which come into the basic group, but those mentioned are the more common and typical species.

33. Fragmental Igneous Rocks.--All the igneous rocks briefly described above are more or less distinctly crystalline in texture. There is a class of igneous rocks, however, which do not present this character, but when fine-grained are dull and earthy in texture, and frequently consist merely of a rude agglomeration of rough angular fragments of various rocks. These form the FRAGMENTAL group of igneous rocks. The ejectamenta of loose materials which are thrown out during a volcanic eruption, consist in chief measure of fragments of lava, &c. of all sizes, from mere dust, sand, and grit, up to blocks of more than a ton in weight. These materials, as we shall afterwards see, are scattered round the orifice of eruption in more or less irregular beds. The terms applied to the varieties of ejectamenta found among modern volcanic accumulations, will be given and explained when we come to consider the nature of geological agencies. In the British Islands, and many other non-volcanic regions, we find besides crystalline igneous rocks, abundant traces of loose ejectamenta, which clearly prove the former presence of volcanoes. These materials are sometimes quite amorphous--that is to say, they shew no trace of water action--they have not been spread out in layers, but consist of rude tumultuous accumulations of angular and subangular fragments of igneous rocks. Such masses are termed trappean agglomerate and trappean breccia. At other times, however, the ejectamenta give evidence of having been arranged by the action of water, the materials having been sifted and spread out in more or less regular layers. What were formerly rude breccias and agglomerates of angular stones now become trappean conglomerates--the stones having been rounded and water-worn--while the fine ingredients, the grit, and sand, and mud, form the rock called trap tuff. Fragmental rocks are often quite indurated--the matrix being as hard as the included stones. But as a rule they are less hard than crystalline igneous rocks, and in many cases are loose and crumbling. When a fragmental rock is composed chiefly of rocks belonging to the acidic group, we say it is felspathic. When augitic and hornblendic materials predominate, then other terms are used; as, for example, dolerite tuff, greenstone tuff.

STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.

34. The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or beds, which are piled one on the top of the other. Rocks which are so arranged are said to be stratified, and are termed strata. We may also use the word stratum as an occasional substitute for bed. The planes of bedding or stratification are sometimes very close together, in other cases they are wide apart. When the separate beds are very thin, as in the case of shale, it is most usual to term them laminæ, and to speak of the lamination of a shale, as distinguished from the bedding of a sandstone. Planes of bedding are generally more strongly marked than planes of lamination. The laminæ frequently cohere, while beds seldom do. In the above figure, which represents a vertical cutting or section through horizontal strata, the planes of lamination are shewn at l, l, l, and those of stratification at s, s, s. There are hardly any limits to the thickness of a bed--it may range from an inch up to many feet or yards, while laminæ vary in thickness from an inch downwards.

35. Hitherto we have been considering the laminæ and strata as lying in an approximately horizontal plane. Sometimes, however, the layers of deposition in a single stratum are inclined at various angles to themselves, as in the following figure. This structure is called false bedding; the layers or laminæ not coinciding with the planes of stratification. It owes its origin to shifting currents, such as the ebb and flow of the tide, and very often characterises deposits which have been formed in shallow water. (Hillocks of drifting sand frequently shew a similar structure, but their false bedding is, as a rule, much more pronounced.)

36. Mud-cracks and Rain-prints.--The surfaces of some beds occasionally exhibit markings closely resembling those seen upon a flat sandy beach after the retreat of the tide--hence they are called ripple-marks or current-marks. They are, of course, due to the gentle current action which pushes along the grains of sand, and hence, such marks may be formed wherever a current sweeps over the bottom of the sea with energy just sufficient for the purpose. But since the necessary conditions for the formation of ripple-mark occur most abundantly in shallow water, its frequent appearance in a series of strata may often be taken as evidence, so far, for the shallow-water origin of the beds. Besides ripple-marks we may also detect occasionally on the surfaces of certain strata mud-cracks and rain-prints. These occur most commonly in fine-grained beds, as in flagstones, argillaceous sandstones, shales, &c. The mud-cracks resemble those upon a mud-flat which are caused by the desiccation and consequent shrinkage of the mud when exposed to the sun. The old cracks have been subsequently filled up again by a deposition of mud or sand, usually of harder consistency than the rock traversed by the cracks. Hence, when the bed that overlies the mud-cracks is removed, we find a cast of these projecting from its under surface, or frequently the cast remains in its mould, and forms a series of curious ridges ramifying over the whole surface of the old mud-flat. Rain-prints are the small pits caused by the impact of large drops. They are usually deeper at one side than the other, from which we can infer the direction of the wind at the time the rain-drops fell. Like the mud-cracks, they are most commonly met with in fine-grained beds, and have been preserved in a similar manner. Some geologists have also been able to detect wave-marks, 'faint outlinings of curved form on a sandstone layer, like the outline left by a wave along the limit where it dies out upon a beach.'

37. Succession of Strata.--The succession of strata is often very diversified. Thus, we may observe in one and the same section numberless alternating beds of sandstone and shale from an inch or so up to several feet each in thickness, with seams of coal, fireclay, ironstone, and limestone interstratified among them. In other cases, again, the succession is simpler, and some deep quarries shew only one bed, as is the case with certain limestones, fine-grained sandstones (liver-rock), and many volcanic rocks. Some limestones, indeed, shew small trace of bedding throughout a vertical thickness of hundreds of feet.

38. Beds, their Extent, &c.--Beds of rock are not only of very different thicknesses, but they are also of very variable extent. Some may thin gradually away, or 'die out' suddenly, in a few feet or yards, while others may extend over many square miles. Beds of limestone, for example, can often be traced for leagues in several directions; and if this be the case with certain single beds, it is still more true of groups of strata. Thus the coal-bearing strata belonging to what is called the Carboniferous period cover large areas in Wales, England, Scotland, and Ireland, not less, probably, than 6000 square miles; and strata belonging to the same great period spread over considerable tracts on the Continent, and a very extensive area in North America. It holds generally true that beds of fine-grained materials are not only of more equal thickness throughout, but have also a wider extension than coarser-grained rocks. Fine sandstones, for example, extend over a wider area, and preserve a more equable thickness throughout than conglomerates, while limestones and coals are more continuous than either.

39. When a bed is followed for any distance it is frequently found to thin away, and give place to another occupying the same plane or horizon. Thus a shale will be replaced by a sandstone, a sandstone by a conglomerate, and vice versâ. Sometimes also we may find a shale, as we trace it in some particular direction, gradually becoming charged with calcareous matter, so as by and by to pass, as it were, into limestone. Every bed must, of course, end somewhere, either by thus gradually passing into another, or by thinning out so as to allow beds which immediately overlie and underlie it to come together. Not unfrequently, however, a bed will stop abruptly, as in fig. 3.

40. Sequence of Beds.--It requires little reflection to see that the division plane between two beds may represent a very long period of time. Let the following diagram represent a section of strata, s being beds of grit, and a, b, c, beds of sandstone and shale. It is evident that the beds s must have been formed before the strata b were deposited above them. At ×, the beds a and b come together, and were attention to be confined to that part of the section, the observer might be led to infer that no great space of time elapsed between the deposition of these two beds. Yet we see that an interval sufficient to allow of the formation of the beds s must really have intervened. It is now well known that in many cases planes of bedding represent 'breaks in the succession' of strata--'breaks' which are often the equivalents of considerable thicknesses of strata. In one place, for example, we may have an apparently complete sequence of beds, as a, b, c, which a more extended knowledge of the same beds, as these are developed in some other locality, enables us to supplement, as a, s, b, c.

41. Joints.--Besides planes or lines of bedding, there are certain other division planes or joints by which rocks are intersected. The former, as we have seen, are congenital; the latter are subsequent. Joints cut right across the bedding, and are often variously combined, one set of joint planes traversing the rock in one direction, and another set or sets intersecting these at various angles. Thus, in many cases the rocks are so divided as easily to separate into more or less irregular fragments of various sizes. Besides these confused joints there are usually other more regular division planes, which intersect the strata in some definite directions, and run parallel to each other, often over a wide area: these are called master-joints. Two sets of master-joints may intersect the same strata, and when such is the case, the rock may be quarried in cuboidal blocks, the size of which will vary, of course, according as the two sets of joints are near or wide apart. Joints may either gape or be quite close; so close, indeed, as in many cases to be invisible to the naked eye. Certain igneous rocks frequently shew division planes which meet each other in such a way as to form a series of polygonal prisms. The basalt of Staffa and Giants' Causeway are familiar examples of this structure. Jointing is due to the gradual consolidation of the strata, and hence, in a series of strata, we may find the separate beds, according to their composition, very variously affected, some being much more abundantly jointed than others. Master-joints which traverse a wide district in some definite direction probably owe their origin to tension, the strata having been subjected to some strain by the underground forces.

42. Cleavage.--Fine-grained rocks, more especially those which are argillaceous, occasionally shew another kind of structure, which is called cleavage. Common clay-slate is a type of the structure. This rock splits up into innumerable thin laminæ or plates, the surface of which may either be somewhat rough, or as smooth nearly as glass. The cleavage planes, however, need not be parallel with the planes of bedding; in most cases, indeed, they cut right across these, and continue parallel to each other often over a very wide region. The original bedding is sometimes entirely obliterated, and in most cases of well-defined cleavage is always more or less obscure.

In the preceding diagram, the general phenomena of bedding, jointing, and cleavage are represented. The lines of bedding are shewn at S, S; another set of division-planes (joints) is observed at J, J, intersecting the former at right angles--A, B, C being the exposed faces of joints. The lines of cleavage are seen at D, D, cutting across the planes of bedding and jointing.

43. Foliation is another kind of superinduced structure. In a foliated rock the mineral ingredients have been crystallised and arranged in layers along either the planes of original bedding or those of cleavage. Mica-schist and gneiss are typical examples.

44. Concretions.--In many rocks a concretionary structure may be observed. Some sandstones and shales appear as if made up of spheroidal masses, the mineral composition of the spheroids not differing apparently from that of the unchanged rock. So in some kinds of limestone, as in dolomite, the concretionary structure is often highly developed, the rock resembling now irregular heaps of turnips with finger-and-toe disease, again, piles of cannon-balls, or bunches of grapes, and agglomerations of musket-shot. A spheroidal structure is occasionally met with amongst some igneous rocks. This is well seen in the case of rocks having the basaltic structure, in which the pillars, being jointed transversely, decompose along their division planes, so as to form irregular globular masses. In many cases, certain mineral matter which was originally diffused through a rock has segregated so as to form nodules and irregular layers. Examples of this are chert nodules in limestone; flint nodules in chalk; clay-ironstone balls in shale, &c.

45. Inclination of Strata.--Beds of aqueous strata must have been deposited in horizontal or approximately horizontal planes; but we now find them most frequently inclined at various angles to the horizon, and often even standing on end. They sometimes, however, retain a horizontal position over a large tract of country. The angle which the inclined strata make with the horizon is called the dip, the degree of inclination being the amount of the dip; and a line drawn at right angles to the dip is called the strike of the beds. Thus, a bed dipping south-west will have a north-west and south-east strike. The crop or outcrop (sometimes also, but rarely, called the basset edge) of a bed is the place where the edge of the stratum comes to view at the surface. We may look upon inclined beds as being merely parts of more or less extensive undulations of strata, the tops of the undulations having been removed so as to expose the truncated edges of the beds. In the following diagram, for example, the outcrops of limestone seen at l, l, are evidently portions of one and the same stratum, the dotted lines indicating its former extent. The trough-shaped arrangement of the beds at s is called a synclinal curve, or simply a syncline; the arched strata at a forming, on the contrary, an anticlinal curve or anticline.

When strata shew many and rapid curves, they are said to be contorted. The diagram section (fig. 10) will best explain what is meant by this kind of structure.

46. In certain regions, the strata often dip in one and the same direction for many miles, at an angle approaching verticality, as in the following section. It might be inferred, therefore, that from A to B we had a gradually ascending series--that as we paced over the outcrop we were stepping constantly from a lower to a higher geological horizon. But, in such cases, the dip is deceptive, the same beds being repeated again and again in a series of great foldings of the strata. Such is the case over wide areas in the upland districts of the south of Scotland. The section (fig. 11) shews that the beds are actually inverted, the strata at × × being bent back upon strata which really overlie them.

47. Contemporaneous Erosion.--Occasionally a group of strata gives proof that pauses in the deposition of sediment took place, during which running water scooped out of the sediment channels of greater or less width, which subsequently became filled up with similar or dissimilar materials. The diagram (fig. 12) will render this plain. At a we have beds of sandstone, which it is evident were at one time throughout as thick as they still are at × ×. Having been worn away to the extent indicated, a deposition of clay (b) succeeded; and this, in turn, became eroded at c, c, the hollows being filled up again with coarse sand and gravel. In former paragraphs, we found reason to believe that lines of bedding indicated certain pauses in the deposition of strata. Here, in the present case, we have more ample proof in the same direction.

48. Unconformability.--But the most striking evidence of such pauses in the deposition of strata is afforded by the phenomenon called unconformability. When one set of rocks is found resting on the upturned edges of a lower set, the former are said to be unconformable to the latter. In the above section (fig. 13), a, a, are beds of sandstone resting on the upturned edges of beds of limestone, shale, and sandstone, l, s. Figs. 14 and 15 give other examples of the same appearance. It is evident that, in the case of fig. 14, the discordant bedding chronicles the lapse of a very long period. We have to conceive first of the deposition of the underlying strata in horizontal or approximately horizontal layers; then we have to think of the time when they were crumpled up into great convolutions, and the tops of the convolutions (the anticlines) were planed away: all these changes intervened, of course, after the lower set was deposited, and before the upper series was laid down. In the case represented in fig. 15, we have a double unconformability, implying a still more elaborate series of changes, and probably, therefore, a still longer lapse of time.