On the eastern side of Barnavave, at the immediate junction of the igneous mass with the surrounding limestone and about a quarter of a mile south of the fault which crosses the mountain, there is an isolated outcrop of a fine-grained, speckled rock which clearly invades the limestone at this point. The rock includes xenoliths broken from the limestone and also others of igneous origin.-and smaller dimensions. Microscopical examination proves the rock to be a syenite of unusual character which carries a small amount of quartz. It is possibly the rock referred to by Harker.

In a recent paper (Nockolds, 1946) it was shown that certain granite-aplites associated with acid plutonic rocks of Caledonian age in Western Scotland fell on, or were very close to, the plagioclase-potash felspar-quartz cotectic curve (the granitic cotectic curve), the approximate position of which could be determined from experimental data now available. It seemed of interest to determine whether other granite-aplites from other areas behaved in the same way and chemical analyses of granite-aplites and related rocks were collected from the literature with this end in view.

In a recent paper the author attempted to explain something of the mechanism of contamination in acid magmas, but nothing was said about the tendency, of contamination to produce normal rock types. It is to repair this deficiency that the present communication is presented.

This is the first of a series of contributions dealing with various aspects of the petrology of Barnavave Mountain which have not been touched upon by earlier investigators. The general relationships existing between the various rock types on Barnavave were first described by Traill in 1878, though some of the rock types had been previously dealt with by Haughton. This was followed by the now classical work of Sollas on some of the hybrids occurring on the western side of the mountain. These are at present being redescribed by Dr. G. D. Osborne in a paper which will appear shortly. More recently, the Carlingford area as a whole has been dealt with by Richey and Osborne has described the metamorphosed limestones of the region. Many of the rock types, however, have never been described in detail, if at all, and it is with these that the present contributions are concerned. The author wishes to express his thanks to Prof. W. J. Pugh, Prof. C. E. Tilley and Dr. G. D. Osborne for facilitating the work in various ways, to acknowledge a grant from the Worts fund and a Royal Society grant which defrayed the cost of two of the analyses. The field work was carried out in 1932 but unforeseen circumstances prevented the earlier working up and publication of the results.

I had not intended to reply to Professor Holmes's recent paper in the Geological Magazine, but the apparent confusion of mind which exists about the question of contrasted differentiation has led me to do so.

WHILST studying the Bibette Head Granite, the writer had occasion to devote a certain amount of attention to the rocks with which it is in contact. In the region under consideration, these are essentially of dioritic type. The Orbicular Diorite was first found in a small embayment forming part of the west side of Corblet's Bay. It was found later in a less well developed fashion, at Saye Bay. It is quite possible that extended search would prove the presence of more. For the most part the orbicules are widely scattered, but a boulder was found at the Corblet's Bay locality which shows several associated with inclusions of a more basic rock. This boulder is now in the Sedgwick Museum, Cambridge.

The limestone xenoliths about to be described were collected on the eastern side of Barnavave from a disused quarry situated at the junction between the “junction hybrids” and the surrounding Carboniferous limestone. The material comes from loose blocks left over during quarrying operations. Both the hybrids and the metamorphosed limestones have been dealt with in detail elsewhere1 and will only be referred to incidentally here.

THE Bibette Head granite outcrops along the western portion of the north coast of Alderney. The outcrop, as seen at present, is about five-eighths of a mile long in an E.-W. direction and about one quarter of a mile wide. The greater part of the intrusion must, however, be under the sea. The rocks into which this granite is intruded are a rather variable series of diorites, and the contact is a sharp one, although there is a certain amount of veining by the granite. Over a large part of the outcrop the granite is much decomposed and reddened. This would appear to be due largely to the action of iron-bearing solutions, derived from the magma, coming up along fissures which are parallel to the main joint planes. These run in a direction 17° west of north. The same series of solutions were probably responsible also for the epidotization of the granite which is a marked feature in various parts. Recent blasting operations at Bibette Head have exposed a large quantity of fresh granite, and although specimens were collected from all over the outcrop the bulk of the material with which the present investigation is concerned came from there.

The rocks about to be described are found in various parts of the eastern and south-eastern portions of the eucrite mass which forms the bulk of Barnavave Mountain. They are best displayed, however, between the 700 and 800 foot contours in the area just north of the large N.W. fault which crosses the region.1 The main characters of the normal eucrite are already well known from Sollas's now classical researches on the western side of the mountain.2 The normal eucrite of the eastern part of Barnavave is very similar to that occurring in the west. It is composed essentially of a diallagic pyroxene and a basic plagioclase, with or without a small but variable quantity of rhombic pyroxene (Text-fig. 1A). The plagioclase varies from Ab19An81 to Ab30An70, but the bulk centres round the more acid of these extremes. When subjected to metamorphism by the later acid intrusions, it shows very fine “clouding” and may appear black in hand specimen. The diallagic pyroxene is almost colourless in thin section but has a pale brown colour in thicker pieces. It has a % 1.680 ±.003, γ % 1.703 ±.003, γΛc % 41°, (+) 2V % 63°±4°, dispersion moderate p>v. Achemical analysis will be found on p. 477. In places it has a well defined schiller structure. The rhombic pyroxene (enstatite), when present, is colourless and non-pleochroic in thin section and also shows schiller structure. The accessory constituents include some titaniferous magnetite or ilmenite (normally enclosed in pyroxene). Apatite appears to be negligible in quantity.

The determination of the true order of crystallization of minerals in a plutonic intrusion is not an easy matter, but a good deal of information on the subject may be obtained if such an intrusion shows a chilled edge or if there are dykes of corresponding composition. Fortunately, some of the Caledonian plutonic rocks of Western Scotland which we are about to consider do show chilled margins, and there is a series of dyke rocks, loosely termed porphyrites and quartz-porphyries, covering practically the whole range of composition for the rocks on the assumed liquid line of descent.1 It has been possible to study large collections of these housed in the Geological Department of Manchester University and in the Department of Mineralogy and Petrology, Cambridge.

An account is given of the mineralogy and chemistry of the ‘pseudodiorites’ of Dielette, the hornfelses from which they are derived, and the contaminated granite veins associated with them.

Some years ago the author gave a short description of a quartz-bearing syenite and its xenoliths occurring on the eastern side of Barnavave. The locality was revisited recently and more material wascollected, while another outcrop of syenite was discovered farther tothe north at locality 5.

The new material collected at the original locality is not very differentfrom that previously described. The syenite occurs as veins in lime-stone and, whereas all the previously described material was quartz-bearing, some of the syenite in very narrow veins is virtually free fromquartz. Again, certain specimens carry a fair amount of late-crystallizing wollastonite which may be replaced to a varying degree by pecto-lite.

The mineral tilleyite was first discovered by Larsen and Dunham in thermally metamorphosed limestones from the contact zone at Crestmore, California. It is associated there with merwinite, wollastonite, spurrite, and gehlenite, and the rock is traversed by streaks of idocrase with grossular. The amount of tilleyite in these rocks was apparently small, but in certain thermally metamorphosed limestones from Carlingford it is abundant and some specimens are practically pure tilleyite-rocks, it has been possible, therefore, to make a more detailed optical and chemical examination of this mineral. The tilleyite occurs as subidioblastic crystals when abundant, or as rounded grains in calcite-rich rocks, or as irregular plates.

The Caledonian plutonic rocks of Western Scotland form a well-defined group, ranging from ultrabasic to acid types, which have been studied in detail by a number of petrologists. They form intrusions of varying size, usually composite and frequently, at least, having the habit of ring complexes. The rock types include peridotites and pyroxenites, gabbros, hornblendite and appinite, appinitic diorites, pyroxene-mica diorites, hornblende-biotite diorite, various hornblende-biotite grandiorites, biotite adamellite and muscovite-biotite adamellite, while the final stage is represented by numerous veins of aplite and rare pegmatites.

The existence of an unidentified mineral in the riebeckite-microgranite of Mynydd Mawr has been known for almost exactly fifty years. It was first noticed by Harker and described by him in the following words: ‘The ground-mass [of the microgranite] is a finely granular admixture of quartz and orthoclase, as in an ordinary “microgranite”, but containing in addition another mineral which is often plentiful. It occurs in minute crystals of acicular or rectangular form, scattered through the ground-mass or included by the other constituents, and having usually a fluxional arrangement agreeing with that of the parallel hornblende and tourma]ine streams. The mineral is colourless, or in the larger crystals gives for vibrations parallel to the long axis a faint tint of indigo-blue. The crystals give high polarisation colours and straight, or nearly straight, extinction. They have a high refractive index, which causes them to stand out in relief when viewed by ordinary transmitted light.’

In the memoir dealing with the geology of the Broken Hill district, New South Wales, mention is made 1 of a remarkable rock type found in a few places in the vicinity of Broken Hill, which looks not unlike an aplite in the field. A brief description of its appearance in thin section is given. It consists mainly of what appears to be very basic plagioclase. A fair amount of quartz, some epidote and bluishgreen hornblende were noticed, whilst a notable amount of sphene is present.

The Dhoon granite is contained in sheets 8 SW. and 8 SE. of the 6-inch Ordnance Survey Maps of the Isle of Man. It is a bosslike intrusion of irregular outcrop (fig. 1), enclosing an area about one and one-third miles long in an east-west direction, and half a mile wide in a north-south direction. It has been intruded into rocks which form part of that division of the Manx slates known as the Lonan Flags, and has metamorphosed these to a certain extent.