Rocks and Their Origins

by Grenville A. J. Cole

GNEISSES

Gneisses may be broadly defined as banded crystalline rocks in which felspar is visible to the unaided eye. Though this will include many igneous masses, it is doubtful if a more rigid description can be given. Numerous gneisses, in fact, owe their parallel structures to flow while in a molten state. Others are rocks that have been deformed by pressure, and their constituents have become drawn out along planes of solid flow. Where actual shearing has taken place, the minerals in the close neighbourhood of the planes of movement may become especially modified, ground down, and deformed. The foliated structure may then be marked by the appearance of differentiated bands. Such bands may also arise from the spreading out under pressure of certain large constituents, such as porphyritic crystals of felspar, which produce white bands, or of pyroxene, which will become modified into granular amphibole and will produce dark streaks through the rock.

Fig. 19. Composite Gneiss. Gartan Lough, Co. Donegal. Fragments of mica-schist project from a gneiss, the banding of which follows the foliation planes of the schist. On the right the mass retains less schist and is more granitic.

Gneisses may also result from the intrusion of felspathic igneous rocks, in sheets of varying thickness, between the layers of a sediment or a schist ([Fig. 19]); or from the intrusion of one igneous rock into another, with varying degrees of interaction and absorption.

It has often been presumed that the invaded igneous rock must have been in such cases in a plastic state. The supply of heat within the earth during such processes, and the action of the gases, corroding, as Doelter says, "like a blowpipe-flame," are, however, clearly sufficient to melt down large blocks, the residue being then carried forward as wisps or bands in the invader.

Many strikingly banded gneisses are thus of composite origin. Their felspathic granitoid bands can be traced in the field to an igneous source, while their darker and usually micaceous layers can as surely be attributed to the invasion and incorporation of adjacent schists ([Fig. 20]). But it is quite possible that in other cases the banded gneiss is a sedimentary rock which has undergone what Judd[103] has styled "statical metamorphism." The differences in successive bands are then due to original differences in successive strata; one has yielded a granitic layer, one a layer of quartzite, one, which was more argillaceous, a layer of mica-schist. The bands in such a gneiss record the stratification.

Fig. 20. Composite Gneiss formed by intrusion of granite into hornblende-schist. Ängnö, near Saltsjöbaden, Sweden.

Gneisses are often described as if they consisted of layers of various minerals, quartz, felspar, and mica, alternating one with another. As a matter of fact, a gneiss may exist in which there is no differentiation into layers; the whole of the constituents have been drawn out and elongated, any mica present becoming naturally conspicuous by its flattened wisp-like forms. The banded gneisses, on the other hand, where layer-structure is obvious, consist in reality of bands of different rock-types. Sometimes all the layers are granitoid, but one band will contain only quartz and felspar, while another will contain the same minerals with an admixture, and perhaps a great predominance, of mica.

G. P. Scrope[104] made an immense step forward when he realised in 1825 that such banded rocks, "the inferior crystalline zones," might be pushed out of position and "protruded" among others "in a solid or nearly solid state." He goes on, "The protrusion of the foliated rocks, gneiss, mica-schist, clay-slate, etc. was chiefly occasioned by their peculiar structure; the parallel plane surfaces of their component crystals, particularly the plates of mica, sliding with facility over one another; while the laminar structure of these rocks was in turn increased during this process, the crystals being elongated in the direction of their motion, as in the case of the clinkstones and pearl-stones of the trachytic formation." After this, there was little left for the later advocates of dynamo metamorphism to put forward.

While Darwin[105] recognised how the granite at Cape Town had worked its way insidiously between the layers of a schist, it was left for Michel Lévy to emphasise the part played by what is called lit-par-lit injection in the making of banded gneiss (see [p. 120]). K. A. Lossen, Johann Lehmann, and other distinguished workers in Germany made clear, on the other hand, the effects of pressure in moulding and reforming crystalline rocks, and even in bringing about the crystallisation of certain minerals in a previously sedimentary mass.

The dynamo-metamorphic school assumed immense importance from 1884 onwards, the date of the publication of Lehmann's work on "Die Entstehung der altkrystallinischen Schiefergesteine," and for a time the intrusion of igneous masses was held, both in Germany and the British Isles, to have had a merely local significance as a metamorphic agent. Wherever "regional metamorphism" was spoken of, pressure-effects were held to be predominant. Indeed, the profound modifications that may occur in rocks when lowered into subterranean cauldrons is only now becoming generally realised. The tendency to regard the structures of large masses of gneiss as of necessity due to deformation and shearing in a solid state has, however, passed away[106].

Pressure-effects are of course clearly traceable in most gneisses, and are of immense importance in many metamorphic areas; but we find again and again that gneissic structure has been injured rather than developed by crushing subsequent to the consolidation of the rock. In some cases, where this structure is due to igneous flow, which of course often took place under considerable pressure, even the puckerings of the stratified or foliated rock which was invaded by the igneous magma have been followed by the invading sheets. In other cases, as in the composite amphibolite gneiss of Canada, or the similar rocks of the Ox Mountains in Ireland, the contortions in the mingled mass are clearly due to the viscid flow of the consolidating invader.

The growing appreciation of the views on recurrent thermal metamorphism that were originally propounded by James Hutton in 1785 has led to the assignment of far younger ages to many masses previously regarded as "fundamental" and Archæan. Some of these rocks are undoubtedly of high antiquity, but are found to be intrusive in strata of a late pre-Cambrian series. Others, such as the material of the Saxon laccolite, and the gneisses on the north-east Bohemian border, are now known to be of Upper Palæozoic age.