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THE STRUCTURE OF ROCK MASSES
BY DAVID FORBES, F.R.S., &c.
T appears extremely probable that one of the first observations I which the beginner in Geology or Physical Geography would make, upon commencing in the field the study of the rock masses which compose the exterior of our globe, would be to notice that rocks are not mere amorphous or structureless aggregations of mineral matter, disposed at random like rubbish shot out of a cart, but that, as a rule, they possess some definite structure or internal mechanical arrangement of the particles of which they are built up; a little further observation will then point out that such an internal arrangement may, frequently at least, be altogether independent of the external configuration of the rock masses themselves.
It will next be perceived, that such rock structure may differ greatly in character in the different classes of rocks, and the conclusion will assert itself that, of all the varieties of structure met with in the field, those in which the particles show an arrangement in more or less parallel lines along which the rock can usually be more easily divided or split up than in other directions, are by far the most frequent in occurrence.
A still minuter consideration of parallel structure teaches that it may be classified under five distinct heads, to which the terms stratification, joints, cleavage, foliation, and striation *
• The term "striation” or “vitreous striation” is here employed to signify the parallel arrangement which is entirely due to the development of what are called the “striæ " of fusion, such as are seen in glass, furnace slags, obsidian, vitreous lavas, glacial ice, &c. This structure owes its origin to the effects of the (unequal rate of ?) movement in the different layers of a substance whilst in a more or less vis id condition, in the act of consolidating from a state of liquidity.
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have been respectively applied; the first and third of these, stratification and cleavage, are only encountered in rocks of
rocks, whether sedimentary or eruptive, is, it must be remarked, not always, or even necessarily, a parallel structure, yet it is so often such, that, in order to avoid confusion, it becomes necessary to consider it also under this head; the fourth, foliation,
and likewise occurs in certain eruptive rocks; whilst the fifth or last, striation, is entirely confined to such rocks as have solidified from a previous condition of liquidity or fusion.
Since to attempt more would be to exceed the space at disposal in a single number of this Review, it is proposed upon the present occasion to treat only of the three first of the above-named rock structures, namely, stratification, joints, and cleavage.
Stratification, or, as it is very commonly called, bedding, is the term employed by geologists to denote a parallel structure in rocks caused by the successive subaqueous deposition of layers more or less thick of mineral matter previously held in solution or suspension in water, and it might even be said in air, for, upon a smaller scale, and in more exceptional instances, stratification may be of subaërial origin, as for example, in the cases of beds frequent in volcanic countries, consisting of alternating layers or beds of volcanic ashes, sand, and still coarser scoriæ, which, after having been thrown up from the crater high up into the air, are spread out by the action of the winds over the neighbouring country, often to a considerable distance ; occasionally also, the action of the winds upon loose surface sands, like those in the desert, produces an arrangement of the particles analogous to subaqueous stratification.
Although the planes of stratification are usually spoken of as parallel, this is not strictly true, for in reality they always have a gentle slope upwards from the sea towards that part of the land from which, by the action mainly of river currents, they have derived the rock débris, out of which the strata themselves are formed. Sedimentary beds are very frequently found to die out or thin out like wedges, which sometimes is owing to a deficient supply of like material, but probably more often to the disturbing effects of currents, &c.
Regarded on the large scale, however, stratification possesses all the general features of parallelism.
It not unfrequently happens, particularly in the case of sandstones, that, independent of the usual parallelism of the beds of deposition to one another, another series of more or less parallel lines may be seen developed within the particular beds themselves, and covering them diagonally at a greater or less incliation to the true plane of sedimentary deposition : this is commonly called “cross or false” bedding, and is usually ascribed to the disturbing action of currents during the period of the formation of the strata ; what has by Sorby been called “ripple drift” is an analogous structure. In the same mass of rock such false bedding may sometimes be seen inclined in opposite directions, in beds superposed one above another, a result probably due to alterations in the direction of the current in the same locality, such as the ebb and flow of the tides.
In most cases the planes of stratification in large rock masses can be traced with ease, owing to the occurrence of alternating beds differing in mineral character, such as those of argillaceous, arenaceous, or calcareous nature; when fossiliferous, the organic remains are a sure guide, since they naturally rest upon the planes of bedding which formerly were the sea bottom, and wben such anciently formed the margin or shores of seas or lakes, it is not uncommon to find ripple marks, foot-tracks of animals, marks after rain drops, and cracks formed by the heat of the sun, causing the mud to contract in drying between the reflux and influx of the tides, all of which are so many proofs of the origin of this structure. Certain strata, such as the limestones, the chalk with its flint nodules and bands, the infusorial siliceous beds, and some of the iron-stone beds, have assumed the form of sedimentary strata, not by the mere mechanical deposition of mineral matter held temporally in suspension in the water, but by the agency of organic life in extracting the lime, silica, or iron, previously held in a different state of chemical combination and solution in sea or fresh water: abundant examples of this action still going on before our eyes are seen in the formation of the great coral reefs, the Atlantic mud, which is but recent chalk, and the lake iron-stones, which last have only lately attracted any attention, and are as yet very imperfectly described.
Other strata, as for instance the beds of rock salt, gypsum, and anhydrite, have no doubt been originally formed simply by the evaporation of inland seas or lagoons of salt water, previously holding these substances in solution.
In some rocks, more especially those pertaining to the older formations, in which, as yet, few or no fossil remains have been found, it is often a matter of considerable difficulty to determine correctly the lines of original sedimentary deposition. In such cases, dependence must be placed mainly on the observation of such minor differences in mineral character and structure as usually present themselves, even in the thicker masses of sedimentary rocks, and in the case of extremely homogeneous deposits, such as, for example, some of the great beds of clay