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THE STRUCTURE OF ROCK MASSES
(FOLIATION AND STRIATION).
BY DAVID FORBES, F.R.S.
IN a previous communication on this subject * the three I structures, stratification, joints, and cleavage, were described ; it now remains to take into consideration the two other systems of parallel structure of common occurrence in the metamorphic and eruptive rocks, viz. foliation and striation.
Foliation. The terms foliated rocks and foliation appear to have been introduced into geological nomenclature some twenty-five or more years ago, by Darwin, and since then they have met with very general acceptation by geologists both abroad as well as at home; by the term foliation is signified, such parallel structure as makes its appearance in rock masses owing to the arrangement of certain crystallised minerals in more or less parallel lines, along which their crystals lie on their flat sides or lengthways, i.e. having their longer axes in the direction of, and not against, the grain of the rock.
All foliated rocks come under the definition of metamorphic rocks, that is, rocks which subsequent to their consolidation have undergone a change in the molecular arrangement of their original component mineral particles, which change in many, if not most instances, has been at the same time accompanied by a re-arrangement of their chemical elements also.
The subject of foliation is one of the most intricate problems in geology, and opinions are much divided as to the signification and origin of these lines of parallel structure, so that, in the present communication, the views propounded must not be regarded as representing any universally accepted doctrine,
: POPULAR SCIENCE REVIEW for April 1870. VOL. X.—NO. XL.
but rather as the conclusions arrived at by the author, after a prolonged study of this subject in the field and in the laboratory.
Although, as before mentioned, all foliated rocks are metamorphic rocks, these may have originally been either of sedimentary or eruptive nature; for a long time it was supposed that foliated structure was alone characteristic of the so-called crystalline schists, until further inquiry into the subject showed that, besides being common in the crystalline or so-called primitive limestones, it was far from being of rare occurrence in many of the plutonic and volcanic eruptive rocks, abundant examples being met with of foliated granites, syenites, gabbros, trachytes, lavas, &c., in which a distinctly recognisable parallel structure is developed by the manner in which one or more of the crystalline mineral components are disposed in the mass of the rock.
The parallelism of foliation is not, as in the case of the other structure already described (stratification, joints, and cleavage), brought about by the formation of divisional planes due to the effects of purely mechanical forces, but, on the contrary, is invariably determined by the presence of crystallised minerals, usually in alternating layers, very different from one another both mineralogically and chemically, and which, owing to the peculiar nature (habit, or behaviour, as it has been called by mineralogists) of the minerals themselves, and to the pressure to which they have been subjected, when in the process of formation or crystallisation, by the weight of the superincumbent mass above them, most commonly assume the form of crystallised foliæ, or of crystals developed mainly in the direction of one of their axes only. From this it will be seen that it is very easy to discriminate between foliation and all other parallel structures likely to be encountered in rock masses. If at times (as in the coal formation, for example) we find minor sedimentary beds, made up almost entirely of plates of mica, or sandstones possessing a fissile or laminated structure from numerous scales of mica which may be arranged in more or less parallel lines, a closer examination of the rock, and more particularly of the mica in it-using the microscope if necessary—will at once show, that it has been deposited as a sediment from water, as the particles will be found waterworn and abraded, and to present an appearance totally different from that of the foliæ, crystallised in situ, which are met with in the crystalline schists or other true foliated rocks.
The simplest form of foliated rocks which occur in nature are those beds of crystalline schist, solely composed of one mineral, such as many of the mica, chlorite, talc, or hornblende schists; in these, as shown in Pl. LXXIII., fig. 4, the rock is seen to be a mere aggregation of imperfectly developed crystals
of the mineral mostly lying, or, as it were, drawn out in one direction; in fact, the metamorphism in this case appears to consist merely in a re-crystallisation of the same mineral previously present in an amorphous or comminuted condition, without any true chemical action having necessarily been called into operation. When, however, instead of the beds being composed merely of one mineral, we find two of quite different nature-as, for example, mica and quartz—these two minerals, not being capable of reacting upon one another, except at such high temperatures as we have no reason to believe necessary for the development of foliation in rocks, usually segregate or separate from one another, so as to arrange themselves in more or less definite or distinct alternating layers, which, if the rock had been contorted or crumpled up by pressure, may often present the most fantastic appearances; as, for example, in the mica schists in Anglesea, a fragment of which is depicted in Pl. LXXIII., fig. 8.
Where one of the minerals has, for example, like garnet, a very great tendency to assume its crystalline form, we often find numbers of nearly, if not quite perfect dodecahedral crystals of this mineral, enveloped in the foliæ of mica or talc, which curve round them and compose the mass of the rock; the crystals of those minerals which possess an elongated crystalline form, like andalusite, actinolite, kyanite, rhetizite, &c., arrange themselves, as a rule, lengthways between the layers of mica, chlorite, &c.
Many minerals which have less tendency to take the form of perfect crystals, appear, as it were, to segregate out in the form of lenticular or oval nodules, arranged with more or less regularity in the schist; a very perfect example of this is seen in the annexed woodcut, which represents a fragment of di
chroite schist, consisting of a mass of mica (with a little talc), enclosing innumerable nodules of dichroite of a whitish or bluish-white colour, sometimes exhibiting the characteristic play of colours. This specimen is from the borders of Ongsteens Vand in southern Norway, where this rock extends over a considerable area, and as the nodules are pretty uniformly of about the size of a walnut, and the foliation of the mica binds itself around them, the rock itself presents a very pecu
liar appearance, from the immense number of the nodules and the regularity of their arrangement.
The more ancient limestones, formerly called primitive, which are usually more or less crystalline in texture, are frequently found to be foliated with chlorite or mica (as at Inverary), with augite and scapolite (as in the Hebrides), or with spinel and chondrodite (as at Christiansand, in Norway); other minerals might also be mentioned as inducing such structure in the metamorphic limestones. In some instances the foliation of different parts of the same large mass of crystalline limestone may be brought about by minerals very different in character to one another, as shown in the annexed figure.
This woodcut represents a section of a limestone quarry, near Christiansand, in Norway, in which A denotes the overlying granite ; a, b, c, different varieties of crystalline schists in irregular patches; d, coarsely crystalline white limestone or marble; e, crystalline white limestone, foliated by small crystals of augite and scapolite; f, ditto, foliated by mica. Although in this section we find a somewhat confused arrangement of the different rocks, it is seen that the general direction of the lines of foliation remains constant, quite independent of the different character of the minerals by which they are expressed, or the nature of the rock which they traverse.
It must not be supposed, however, that the foliated structure in rocks is only developed by mineral silicates, as in the instances hitherto referred to; on the contrary, other and most distinct compounds frequently make their appearance : thus
when sedimentary strata, which have originally contained carbonaceous matter, have undergone metamorphic alteration, it not unfrequently happens that the carbon becomes converted into graphite, and thus gives rise to graphitic schists, which sometimes assume the exact appearance of ordinary mica schist, composed of alternate layers of quartz and mica ; the latter mineral being in this case represented by the scales of graphite.
Metallic foliation, i.e. foliated structure developed by the occurrence of metallic oxides, sulphides, sulpharsenides, &c., is common in many countries, extending at times over considerable areas. Thus we find in Scandinavia, Brazil, India, and elsewhere, ferruginous schists, in which the mica is replaced by iron glance or the micaceous form of the sesquioxide of iron. The quantity of oxide of iron in these beds of iron schists, as they have been termed, sometimes increases so much as to preponderate over the stony matter, and form actual beds of iron ore often of great magnitude. In like manner we find, in several parts of Sweden, schists foliated with zincblende to such an extent as to be largely worked for zinc ore, as at Arkersund, Shyshyttan, Bovallen, &c.; whilst at Vena, in Sweden, and Modum, in Norway, similar schists occur (and are worked) containing cobalt ore and other arsenical compounds.
Beds of crystalline schists, foliated or impregnated with iron and copper pyrites and other sulphides, are common enough in many countries, and especially so in Scandinavia, where, from the rusty colour assumed by these rocks along the line of outcrop, they are called fahlbands.
With regard to the origin of the crystalline schists, the evidence seems in favour of the view that they are merely sedimentary beds of sand; arenaceous, micaceous, and other muds; and submarine tufts of eruptive origin, altered by crystallisation, or what may be termed chemico-molecular action. Occasionally an examination under the microscope will reveal the contours of the original sand grains, and in some instances, as Sorby has shown, the existence of still unobliterated current structure, like ripple-drifts for example. The argillaceous shales and slates in some parts of Estramadura, in Spain, are seen to be converted into a variety of mica schist, when fragments are found surrounded by or enclosed in the granite. Some of the hornblende schists of southern Norway, which consist almost entirely of crystals of greenish-black hornblende, were long ago shown by the author to be the beds of tuff which had proceeded from the submarine eruptions of pyroxenite (highly augitic trap) in the vicinity, subsequently consolidated and re-crystallised in situ.
When, in addition to the mica and quartz, felspar is also present in these rocks, they receive the name of gneiss, or more