Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/329

Rh META.IORI’HIC Roexs.] are held to be crystalline transformations of ordinary sedi- mentary strata. The problem whetl1er or not certain masses of gneiss or schist represent equivalent unaltered sedi1nent- ary rocks of other districts is mainly one of structural geology. It must be decided by the geologist rather than the chemist. It has been answered in the affirmative by the great majority of stratigraphical geologists all over the world. We may not be entitled to assert that every mass of gneiss or schist is a metamorphosed sedimentary rock. Possibly some foliated rocks of extreme antiquity may have originated directly from chemical precipitation. But when it can be shown that ordinary stratified rocks have been converted into schist and gneiss, it seems permissible to hold that all such rocks have had a similar origin, at least until good reasons can be adduced against that view. It is evident that if the so—called metamorphic rocks can anywhere be seen to graduate into unaltered strata, it is there that they ought to be speciall y studied, and that light may be expected to be cast on their origin and history. It is customary to speak of metamorphism as being either lu._':1l or regional, that is, being conﬁned to one limited p ortiou of a. mass of rock, or as extending throughout the whole of the rock and over wide districts. It is doubtless from the first of these developments that we may hope to learn most regarding metamorphic changes. Loml (C'ou(«.zcl) Jletmzzorp/tism.—The simplest and most obvious cx;unples of thiskind of alteration oceurwhere a mass of igneous rock has invaded sedimentary strata, which have in consequence been affected in lithological character along the margin of contact with the intrusive rock. Allusion has already been made to changes of this kind effected by intrusive sheets and dykcs, and likewise traceable round the edges of ancient voleanie vents. Sandstones are con- verted into a lustrous crystalline quartz-rock; shales and argillaceous strata are indurated into porcellanite, jasper, tlinty-slate, or some analogous product. Limestones are male crystalline; coals are charred, turned into anthracite, and sometimes even into gr-.1phite. These n1etamorphisms extend to very variable distances from the intrusive rock. Sometimes they are scarcely perceptible at all, and disap- pear within a space of a few inches. I11 other cases they exten-d for some feet, and in the case of some coal—seams even to 50 yards or 111ore. It is around bosses of granite however that the most marked examples of local metamorphism can be observed. On a previous page some account has been given of the way in which the Silurian rocks of Ireland and the south of Scotland are pierced by large masses of granite, and how, as they:approach the granite even at a horizontal distance of a mile or more, they begin to assume a. micaceous foliated texture which becomes n1ore and more marked until, along the margrin of the granite, they pass into true mica—schist and gneiss. The identity of origin between these schistose m-1:<ses and the greywackes and shales beyond the metamor- phic zone does not admit of any question. The unaltered rocks can be followed step by step into and through the stages of alteration, until they are found to have acquired the genuine gneissie or schistose structure. Moreover, it may be observed that all the bands of sedimentary rock do not give rise to the same kinds of metamorphic products. Some kinds of strata are more prone to alteration than others, and give rise to more perfect schists. In the south of Scotland, for example, certain greywackes and grits formed of a granular mixture of quartz-sand, felspar, clay, and various decomposition products l1ave been metamor- phosed into perfect mica—schist, while some bands of black anthracitic and graptolitic shale have merely been intensely mdurated and shattered. Many districts in Scotland, in Ireland, in the lake country of England, and in ‘Vales might GEOLOGY 315 be cited as furnishing examples of this gradual conversion of ordinary sedimentary rocks into true schists. Regional xlletamo7y)/zi.sm.——-If then it can be proved that over limited areas thoroughly foliated rocks have been produced by the transformation of ordinary sedimentary strata, a presumption is established in favour of a similar mode of origin for foliated rocks elsewhere and over wider regions. There occur many vast spaces of the earth’s surface occupied by foliated rocks. In Finland, Scandi- navia, and the Scottish Highlands tracts many thousands of square miles in extent consist of gneiss, mica—schist, hornblende—rock, and other members of the same great family of rocks. A large portion of British North America lies upon similar mineral masses. Rocks of this type com- monly rise also along the core of great mountain ranges, as in the Alps and in the Rocky lllountains. If these rocks which cover such vast areas were originally ordinary sedi- mentary rocks, they must have been metamorphosed, not by 1ncre local protrusions of igneous matter, but by some general process. Grctdcmlons of J[etamorpIu'sm.—We have seen above that the nature and extent of the alteration experienced by rocks have been regulated, not merely by the vigour of the meta- morphic process, but by the composition a11d structure of the rocks themselves. A siliceous sandstone, for instance, containing little or no aluminous or other admixture, seems to be capable of retaining much of its original character, while surrounding or intercalated less purely quartzose beds l1ave been completely changed. It is converted into quartz-rock, but still shows the rounded quartz grains of the original sand. In proportion as the sand l1as been mixed with clay it has produced a rock more susceptible of change. The argillaceous (or magnesian) cement has been attacked, and in the process of change the quartz-sand has been affected. Mica in one or other of its various forms, aluminous or maguesian, has very generally appeared, and in proportion to its development has the foliated structure been made apparent. Hence we may obtain every grada- tion from a quartz-rock or grit into a true mica—schist or gneiss. ' Production of Foliation.—The term “foliation” means the separation of a rock into approximately parallel or lenticular crystalline layers or folia of different mineral composition. It implies a segregation and crystallization of mineral matter along distinct planes. Those who main- tain the original chemical precipitation of the most ancient gneisses and schists believe that the folia mark the strati- ﬁcation of the successive layers of deposit. Those, on the other hand, who hold that all the schistose rocks about the origin of which any satisfactory conclusion can be reached were originally mechanical sediments of ordinary kinds, regard the folia as coincident generally with the stratifica- tion of these sediments. The folia of a. gneiss or schist are crystalline aggregates which along their planes of mutual contact are as it were welded or felted into each other by the interlacing of their several component crystals. They are destitute, as a rule, of the parallelism, ﬂatness, and persistence so characteristic of stratification. On the contrary they are apt to swell out into thick concretionary aggregations and to die out rapidly ; they exhibit a wavy, crumpled, or puckered arrangement traceable in vast folds on the side of a moun- tain, and yet descending even to such minute corrugation as can only be detected by the microscope. Foliation occurring in altered sedimentary rocks must be due to a crystallization and rearrangement of the chemical constituents of these rocks along certain lines. On a former page (ante, p. 263) it was pointed out that this kind of metamorphism would as a. rule proceed along the lines of stratiﬁcation. Doubtless in the vast majority of cases