Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/330

Rh 316 the planes of foliation are coincident with the general direction of this stratifi'cation. The metamorphic rocks of the Scottish Highlands furnish admirable proofs of this fact. Bands of quartz-rock and schist alternate with each other and with zones of limestone, precisely as beds of sandstone, shale, and limestone do in unaltered formations. Thin seams of pebbly grit with well-rounded water-worn pebbles may be observed running parallel with the folia of a schist, as a seam of fine grit or conglomerate may be seen to do in a series of shales. Nay, even the false-bedding so characteristic of mechanical sediment may be observed among these metamorphic rocks. The metamorphism is not uniform in these regions. Here and there it becomes intensitie-.1, and the rocks assume a thoroughly crystalline and much disturbed aspect. But as they recede from these areas they are found to lose much of their foliated character, and indeed to present such slight traces of meta- morphism that they can be at once recognized as grey- wackes, grits, and slules. .I -Jreover, they have been thrown into anticlinal and synclinal folds, and in these and other larger features of geological structure they (litter in no essen- tial respect from ordinary unaltered strata. Lastly, fossili- ferous limestones containing Silurian shells have been found at their base, so that they have all been formed long after the seas over the area of Britain had been tenanted by living organisms. Professor Sedgwick and Mr Darwin pointed out many years ago that a crystalline rearrangement of mineral matter has in some cases taken place along the planes of cleavage. We have already observed that the water which has been the great agent of metamorphism nmst always have followed the dominant divisional planes of a rock. If these planes were those of cleavage, the foliation would doubtless be produced along them, irrespective of the original laminae of deposit. So long as the rock remained tolerably homogeneous in chemical composition, there seems no reason why foliation along the cleavage should differ in any material respect from that along strati- ticstion. But it may be doubted whether a cleavage foliation could run without sensible and even very serious interruptions over wide areas. For, in the first place, in most large masses of sedimentary matter we encounter alternations of different kinds of sediment, which could not but produce distinct kinds of rock under the in- ﬂuence of metamorphic change. In the second place, cleavage depends for its perfection and continuity on the fineness of grain of the rock through which it runs. While exceedingly perfect in a mass of argillaceous strata, it becomes feebler or even dies out in a coarse sandy or gritty rock. Hence, where foliation coincides with cleavage over large tracts, there will almost certainly be bands, more or less distinct, coincident with the original stratification, and running oblique to the general foliation, like bedding and cleavage, save where these two kinds of structure may happen to coalesce. In a region of intense metamorphism the foliation of the schists may be observed to become here and there some- what indefinite, until it disappears altogether, and the rocks assume a thoroughly granitic character. Between gneiss and granite there is no difference in mineralogical composi- tion ; in the one rock the minerals are arranged in folia, in the other they have no definite arrangement. Gneiss might be called a foliated granite ; granite might be tcrmed a non-foliated gneiss. The two rocks may be observed to graduate into each other. In Abcrdeenshire, for example, the common grey mica-schist and gneiss may be seen to pass insensibly into the ordinary grey granite. In such cases it has been naturally concluded that granite is the ultimate stage of metamorphism. Judged merely from their composition and microscopic structure, an intrusive G E O L O G Y [1v. s1‘m'c'1'i'r..-u... granite connected with igneous protrusions and a metamor- phic granite representing the thorough transformation of stratiticd rock cannot be distinguished from each other. There is thus nothing im )l‘Ul.‘il.l)le in the idea that the same mineral particles may have gone through many suc- cessive cycles of change. We may suppose them to have been originally part of a granite mass, and to have been subse- quently exposed at the surface by enormous denudation. '0rn away from their parent granite they would be washed down with other particles, and spread out under water as parts of sandy or muddy deposits. Buried under a gradual accumulation of sedimentary material thousands of feet in thickness they might be depressed deep beneath the su1'f:u'0, and be thus brought within the inﬂuence of metamorphism. Gradually recomposed, crystallized, and converted into schistose rock, they might be eventually reduced to the condition of granite, and protruded into some of the over- lying less metamorphosed masses in the form of granite veins. Or we may conceive, as already (rmlc, p. 309) sug- gested, that a communication was opened between the granite thus produced and the surface, and that the original mineral particles, whose vicissitudes we have been tracing, were erupted to the surface as part of a stream of lava. Possible illclctinorp/z27s2n of I _r/ncous I-.’ocl's.—.ln most large tracts of foliated rocks there occur masses less distinctly foliated or quite granitoid in texture, formed mainly of hornblende or of that mineral in combination with others. Zones or bosses of hornblende-rock and hornblende-schi.-t frequently appear among gneiss and mica—schist. Varieties of quartz—porphyry occur in a similar way. llands of ﬁne u11ctuous chloritic schists may also often be traced. It is not easy to understand how such rocks, at least those con- taining a large percentage of magnesia, could be produced by the metamorphism of ordinary sediment. The ditliculty may perhaps be removed if we regard them as having originally been igneous rocks, eit.her erupted at the surface or intrusively injected among the surrounding rocks previous to metamorphism. Such mineral masses as varieties of syenite and diorite, rich in hornblende or other magncsian silicates, might have been the original condition of many of the rocks here referred to. The ﬁne magnesian schists might be regarded as having been at first tuffs associated with the lava-form masses. »S'lrm-tm'e of .l[ct(mzorp/do 1.’or:/as in the I"z'clJ.-—As the series of metamorphic rocks ranges from scarcely altered sedimentary strata on the one hand to crystalline amorphous granitic masses on the other, they must obviously possess a great range of structure as parts of the architecture of the solid land. In particular they n1ust under different circum- stances present the features new of aqueous and now of igneous rocks. The most typical form of metamorphism being foliation, we may consider the structure of foliated rocks as the most characteristic. From what has been said above, it is evident that the planes of foliation give the rocks a general resemblance to stratified sedimentary masses. But these planes are seldom so definite and per- sistent as those of stratification. They do not impart to the rocks the same tendency to split up into well-marked parallel beds. On the contrary they are often so felted or welded together, especially in the coarse and most crystal- line gneisses, that they hardly serve as divisional planes at all, but leave the ﬁrm tough rock to split up along other lines. - With care and patience lines of antielinal and synclinal fold may often be traced among foliated as well as among unaltered rocks. But the unravelling of these and other features of structure is much more diflicult than among ordinary stratiﬁed formations. This arises partly from the frequent absence of conspicuous and persistent bands which could be used as horizons in working out geological