Geology Part 2

33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described above are more or less distinctly crystalline in texture. There is a cla.s.s 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 acc.u.mulations, 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 acc.u.mulations of angular and subangular fragments of igneous rocks. Such ma.s.ses 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-Ma.s.sES.

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-ma.s.ses. 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 subst.i.tute 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 _laminae_, 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 laminae 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 _laminae_ vary in thickness from an inch downwards.

[Ill.u.s.tration: Fig. 1.--_st_, sandstone, and _sh_, shale: _s_, lines or planes of bedding; _l_, lines or planes of lamination.]

35. Hitherto we have been considering the _laminae_ 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 laminae 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 p.r.o.nounced.)

[Ill.u.s.tration: Fig. 2.--False Bedding.]

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 coa.r.s.er-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 versa_. 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 pa.s.s, as it were, into limestone. Every bed must, of course, end somewhere, either by thus gradually pa.s.sing 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.

[Ill.u.s.tration: Fig. 3.--Sudden ending of Bed at .]

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 s.p.a.ce 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_.

[Ill.u.s.tration: Fig. 4.--Sequence of Beds.]

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.

[Ill.u.s.tration: Fig. 5.--Beds of Limestone (_a_), Sandstone (_b_), and Shale (_c_), divided into cuboidal ma.s.ses by master-joints.]

[Ill.u.s.tration: Fig. 6.--Columnar Structure.]

[Ill.u.s.tration: Fig. 7.--Bedding, Joints, and Cleavage (after Murchison).]

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 laminae or plates, the surface of which may either be somewhat rough, or as smooth nearly as gla.s.s. 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 ma.s.ses, 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-b.a.l.l.s, 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 ma.s.ses. 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_ b.a.l.l.s in shale, &c.

[Ill.u.s.tration: Fig. 8.--Dip and Strike of Strata.]

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 _ba.s.set 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_.

[Ill.u.s.tration: Fig. 9.--Anticlines and Synclines.]

[Ill.u.s.tration: Fig. 10.--Contorted Strata.]

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.

[Ill.u.s.tration: Fig. 11.--Inversion of Strata.]

[Ill.u.s.tration: Fig. 12.--Contemporaneous Erosion.]

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 coa.r.s.e 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.

[Ill.u.s.tration: Fig. 13.--Unconformability.]

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.

[Ill.u.s.tration: Fig. 14.--Violent Unconformability.]

[Ill.u.s.tration: Fig. 15.--Double Unconformability.]

49. _Overlap._--When the upper beds of a conformable group of strata spread over a wider area than the lower members of the same series, they are said to _overlap_. The accompanying diagram shews this appearance.

An overlap proves that a gradual submergence of the land was going on at the time the strata were being acc.u.mulated. As the land disappeared below the water, the sediment gradually spread over a wider area, the more recently deposited sediment being laid down in places which existed as dry land at the time when the earliest acc.u.mulations were formed.

Thus, in the accompanying ill.u.s.tration (fig. 16), the stratum marked 1, resting unconformably upon older strata, is overlapped by 2, as that is by 3, and so on--all the beds in succession coming to repose upon the older strata at higher and higher levels, as the old land subsided.

[Ill.u.s.tration: Fig. 16.--Overlap.]

[Ill.u.s.tration: Fig. 17.--Fault.]

50. _Dislocations or Faults._--When strata, once continuous, have been broken across, and displaced or shifted along the line of breakage, they are said to be _faulted_, the fissure along which the displacement occurs being termed a _fault_ or _dislocation_. The simplest form of a fault is that shewn in the following diagram, where strata of sandstone and shale, with a coal-seam, S, have been shifted along the line _f_.

The direction in which the _fault_ is inclined[D] is its _hade_, and the degree of _vertical displacement_ of the beds is the _amount_ of the dislocation. Generally, the beds seem to be pulled _down_ in the direction of the _downthrow_, and _drawn up_ on the opposite side of the fault, as shewn in the diagram. Sometimes the rocks on each side of a fault are smoothed and polished, and covered with long scratches, as if the two sides of the fissure had been rubbed together. This is the appearance called _slickensides_. Slickensides, however, may occur on the walls of a fissure which is not a displacement, but a mere joint or crack. A dislocation is spoken of as a downthrow or an upcast, according to the direction in which it is approached. Thus, a miner working along the coal-seam S, from _a_ to _b_, would describe the fault, _f_, as an _upcast_, since he would have to mine to a _higher_ level to catch his coal again. But, had he approached the fault from _c_ to _d_, he would then have termed it a _downthrow_, because he would see from the hade of the fault that his coal-seam must be sought for at a _lower_ level.

Faults are of all sizes, from a foot or two up to vertical displacements of thousands of feet. Powerful dislocations can often be followed for many miles across a country, running in a more or less linear direction.

Thus, one large fault has been traced across the breadth of Scotland, from near St Abb's Head, in the east, to the coast of Wigtown, in the west. Every large throw is accompanied by a number of smaller ones--some of which run parallel to the main fault, while many others seem to run out from this at various angles. Faults are of all geological ages. Some date back to a most remote antiquity, others are of quite recent origin; and no doubt faults are occurring even now. In the following diagram, the strata, _a, a_, have been faulted and planed away before the strata, _b_, were deposited. Hence, in this case, it is evident that if we know the geological age of the beds, _a_ and _b_, we can have an approximation to the age of the fault. If the beds, _a_, be Carboniferous, and those at _b_ Permian, then we should say the fault was _post-Carboniferous_ or _pre-Permian_.

[D] The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.

[Ill.u.s.tration: Fig. 18.--Ground-plan of Large Main Fault and Minor Displacement Fissures.]

[Ill.u.s.tration: Fig. 19.--Faulted Strata covered by undisturbed Strata.]

51. _Metamorphic and Igneous Rocks--mode of their occurrence._--In the foregoing remarks on the structure and arrangement of rocks we have had reference chiefly to the aqueous strata--that is to say, the _mechanically_, _chemically_, and _organically_ formed rocks. We were necessarily compelled, however, to make some reference to, and to give some description of, certain structures and arrangements which are not peculiar to aqueous strata, but characterise many metamorphic and igneous rocks as well. To avoid repet.i.tion it was also necessary, while treating of _joints_, &c., to give some account of certain structures which are the result of metamorphic action. But, for sake of clearness, we have reserved special account of the structure and mode of occurrence of metamorphic and igneous rocks to this place. After what has been said as to the structure and arrangement of aqueous strata, it is hardly needful to say much about the crystalline schists. These the student will understand to be merely highly altered aqueous rocks,[E] in which the marks of their origin are still more or less distinctly traceable.

As a rule, metamorphic strata are contorted, twisted, and crumpled, although here and there comparatively horizontal stretches of altered rocks may be observed. The regions in which they occur are often hilly and mountainous, but this is by no means invariably the case. The greater part of the mountainous regions of the British Islands is occupied by rocks which are more or less altered; the more crystalline rocks, such as mica-schist, gneiss, &c., being abundantly developed in the Scottish Highlands, and in the north and west of Ireland; while those which are less altered cover large areas in the south of Scotland, and in Wales and the north-west of England. Throughout these wide areas the rocks generally dip at high angles, and contortion and crumpling are of common occurrence. The finer-grained clay-rocks also exhibit fine cleavage planes, and are in some places quarried for roofing-slates--the Welsh quarries being the most famous. Here and there, bedding is entirely effaced, and the resulting rock is quite amorphous, and, becoming gradually more and more crystalline, pa.s.ses at last into a rock which in many cases is true granite. The original strata have disappeared, and granite occupies their place, in such a way as to lead to the inference that the granite is merely the aqueous strata which have been fused up, as it were, _in situ_. At other times the granite would appear to have been erupted amongst the aqueous strata, for these are highly confused, and baked, as it were, at their junction with the granite, from which, also, long veins are seen protruding into the surrounding beds. Metamorphic granite, then, graduates, as a rule, almost imperceptibly into rocks which are clearly of aqueous origin; while on the contrary the junction-line between igneous granite and the surrounding rocks is always well marked. The origin of granite, however, is a difficult question, and one which has given rise to much discussion. Some further remarks upon the subject will be found in the sequel under the heading of _Metamorphism_.

[E] Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally a.s.sume a crystalline texture.