Doris L. Reynolds

The superposition of Caledonoid folds on an older fold-system in the Dalradians of Malin Head, County Donegal [Ireland]

In the Malin Head promontory an Older fold-system, with thrusting and folding about N.W.-trending axes, was followed by Caledonoid folding about N.E.-trending axes. The limbs of the Older folds, both major and minor, now lie in the sheet- dip of the Caledonoid folds, the dips of the Caledonoid limbs being equivalent to the plunges of the axes of the Older folds. Most of the S-planes are shared in common by the two fold-systems. The only exceptions are those that outline the closures and rising parts of the Older fold-system, and these form zones in which the S-planes strike at right angles to the Caledonoid trend. By reference to the outcrop-forms in the area described, and with the aid of plasticine models, it is demonstrated that the outcrop-forms of folded folds are readily recognjzable, and that they provide visible evidence of the approximate axial directions and relative ages of the two fold-systems. By comparative study of outcrop-forms on the models and structures seen in sections cut at right angles to each of the fold-axes, it is found that the structures differ radically from those constructed by projecting the outcrop-forms on planes normal to the axes. The “down-the-plunge” method of viewing a geological map thus provides no clue to the actual three-dimensional structure of folded folds.

The granite controversy

This year, the 150th anniversary of Huttons death, we are particularly reminded of the lively struggle of the eighteenth century between Vulcanists and Neptunists; the more so since that struggle finds its counterpart to-day in the divergence of opinion between two schools of petrology whose respective members might aptly be called Magmatists and Transformists. Much of the disagreement between the modern opponents arises from a belief on the part of the Magmatists that Hutton proved the magmatic origin of granite, and, in addition, the disagreement is probably strengthened by a misunderstanding of the views held by Transformists, and by lack of knowledge or experience of the evidence on which they base their conclusions. Indeed, Transformists might be tempted to preface their writings with the following very apposite quotation from Werner (1781):— “I must here present a request to all who would judge of this theory, or communicate their sentiments on it to the public, viz. to begin by reading through the whole treatise, and then to peruse it a second time, with attention. Such a request will not only appear strange to many persons, but even superfluous ; I find myself, however, under the necessity of making it from the manner followed by some individuals with my book on the external characters of fossils. They have often represented me as saying quite the contrary of what I have expressly written. This may have been done by some, through design, but in by far the greater number of instances, it has happened from that work not having been read through ; and in this manner the public has frequently been led into an error, at least for some time.”

The Two Monzonitic Series of the Newry Complex

In a study of the rocks of the eastern end of the Newry Complex, the writer distinguished two contrasted groups of hybrid rocks— (i) the Slievegarron type, and (ii) the Seeconnell type—both of which are intimately associated with biotite-pyroxenite and biotite-peridotite. The Slievegarron hybrids comprise an augite-biotite-diorite series. The typical augite-biotite-diorite (1934, p. 611) is a highly undersaturated rock heteromorphous with certain varieties of orthoclase-basalt and leucite-basanite; it might equally well be described as biotite-essexite-gabbro. No further discussion of these rocks will be undertaken in this paper, but it seems desirable to point out that they should not be confused—as unfortunately they have been—with gabbro-diorite.

On the relationship between "fronts" of regional metamorphism and "fronts" of granitization

Two important papers, relating to “fronts” of metamorphism, appeared in the Bull. Soc. Geol. France during the war years, and may, in consequence, have escaped the notice of many petrologists. The contents of both papers are so closely bound up with the important subject of granitization as to render their review desirable. In the first of these papers, Perrin and Roubault (1941) describe an apparently unconformable boundary between a Triassic conglomerate and an underlying schist. The Triassic conglomerate consists of red quartzite pebbles in a sandy matrix, whilst the schist is a normal low-grade sericite-chlorite-schist of a green colour. Detailed examination of the contact, however, shows that the relations between the two rock types are by no means so straightforward as a cursory examination suggests. Although the contact between the conglomerate and the schist is generally sharp, a close scrutiny establishes the following local but remarkable apparent anomalies: (1) “Xenoliths” of the conglomerate, and isolated pebbles from it, occur within the schist. (2) “Veins” and pods of the schist occur within the Triassic conglomerate. (3) Gradational contacts are sometimes found between the schist and the Triassic sandstone, the schist facies dying out within the space of a few centimetres. On the basis of these observations Perrin and Roubault reach the inevitable conclusion that the metamorphism which gave rise to the schist actually post-dated the deposition of the conglomerate.

Transfusion Phenomena in Lamprophyre Dykes and their bearing on Petrogenesis

The present investigation establishes the following facts:— (1) Xenoliths of vein quartz in a vogesite dyke near Kircubbin, Ards Peninsula, Co. Down, exhibit felspathic replacement-rims. (2) The replacement-rims have developed as a result of the introduction of alk-aluminous emanations, and to a smaller extent of cafemic constituents, into the quartz xenoliths. (3) Rheomorphic leucocratic veins extend from the felspathized rims through the lamprophyre, the veins being essentially similar in type to felspathic dykes which are associated with British Caledonian lamprophyres. (4) Phenomena similar to those of (1), (2), and (3) are developed in the Newmains dyke, Dumfriesshire. (5) Quartz lenticles in potash-enriched hornfels in the vicinity of the Newmains dyke exhibit similar replacement-rims, and thus bear witness to the high degree of mobility of the emanations which brought about the felspathization. (6) Felspathization of xenoliths is shown, by reference to various recorded examples, to be a phenomenon which characterizes British Caledonian lamprophyres. (7) The Caledonian dyke suite: lamprophyre—porphyrite—porphyry is shown to parallel the rock suites developed by processes of metasomatism and rheomorphism in individual heterogeneous intrusions. (8) Ultrabasic types, such as hornblende-peridotite, biotite-peridotite, hornblendite, biotite-pyroxenite, and scyelite, are shown to be the most basic members of the lamprophyre suite. (9) Chemically, the various components (a) of single heterogeneous lamprophyre dykes and (b) of the Caledonian dyke suite as a whole are shown to be expressible in terms of sialic material, alk-aluminous emanations, and cafemic emanations, the latter being directly related to the ultrabasic types listed in (8).

Contact Metamorphism by a Tertiary Dyke at Waterfoot, Co. Antrim

Close to Waterfoot and about one and a quarter miles south of Cushendall, in Co. Antrim, a bifurcating dolerite dyke of Tertiary age outcrops on the shore approximately 750 feet south-west of Red Bay pier (see text-fig. 1). The dyke is intruded into practically horizontal beds of Triassic sandstone which contain abundant fragments and pebbles of schist and quartzite. As shown on the map, text-fig. 1, the dyke bifurcates and encloses a lenticular portion of these Triassic beds. Where unaltered by the dyke, that is in the low cliff adjoining the sea-wall to the south-west of the dyke and in that portion of the enclosed mass adjoining the south-western branch of the dyke, the Trias is a friable yellow sandstone, rich in fragments of schist and quartzite ranging in size from a few inches down to microscopic dimensions. On the south-western side of the north-eastern branch of the dyke, however, a marked change takes place. The Trias here changes in colour from yellow to grey and becomes hard and compact; and, as the dyke is approached, the fragments and pebbles become less distinct and eventually merge into the matrix of the rock. This change begins between eight and nine feet from the contact. Within a foot from the contact the alteration is so intense that the main part of the rock has a compact homogeneous appearance, so that at first glance it resembles an igneous rock with sparse xenoliths of white quartzite. The schist fragments are here represented by dafk patches which merge, often imperceptibly, into the matrix of the rock.

Granite; some tectonic, petrological, and physico-chemical aspects

Granites of the main movement-phase of an orogenic zone form an integral part of the tectonic pattern being characteristically located at the intersection of cross- and main-folds. Here, an older granitic core pierces upwards through the anticlinal vault to form a diapir-fold in which migmatization, synchronous with movement, leads to the evolution of granodiorite. At higher structural levels where pressure is lower K-granites occur. Only at sub-volcanic levels has evidence of melting been found. Recent hydrothermal investigations, by Bowen and Tuttle, show that synthetic haplo-pitchstones which begin to crystallize at minimum temperatures become richer in K as pressure decreases. These results are closely comparable with evidence provided by the sub-volcanic K-microgranites (migmatized Caledonian granodiorite) of Slieve Gullion, but have no application to the granodiorites.