Arthur Holmes

The Problem of the Association of Acid and Basic Rocks in Central Complexes

One of the outstanding problems of petrogenesis at the present time is that offered by the remarkable association of sharply contrasted acid and basic rocks (e.g. granite-gabbro; granophyredolerite; pitchstone-tholeiite; and rhyolite-basalt) in igneous complexes such as those of the British Tertiary Province and those of the great lopoliths of Duluth and Sudbury in North America and the Bushveld in South Africa. The contrast was first recognized in the lavas of Iceland by Bunsen (1) and it led him to the conception that two fundamental magmas, respectively acid and basic, were concerned in the genesis of the igneous rocks of Iceland and similar provinces elsewhere. Bunsens view, however, has had little influence in the development of petrological philosophy. The petrologists of the Geological Survey in this country, and Bowen and others in North America, have assembled a very weighty and reasonable mass of field and laboratory evidence supporting the hypothesis that the acid rocks are residual products arising from the crystallization-differentiation of basaltic magmas. In the recently published Ardnamurchan Memoir (2), for example, it is claimed that as a result of the early extraction from the Plateau Magma of olivine, pyroxenes, basic plagioclase, and iron ores, the residual magma would reach a composition “which would find expression as quartz-doleritic rocks with an acid mesostasis capable of mechanical separation and a separate existence as acid lavas or intrusions” (p. 95). Bunsens two magmas are thus regarded as successive products from a single parent stock.

The Age of Uraninite and Monazite from the Post-Delhi Pegmatites of Rajputana

The age of uraninite and monazite from two pegmatites occurring at Bisundni and Soniani in Rajputana, India, has been determined as between 740 and 865 million years. Comments on the relative ages of the Delhi, Satpura, Aravalli, and Dharwar orogenic belts are included.

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 Petrology of Katungite

The specimens representing the Katunga lavas are compact fine-grained porphyritic rocks of lustrous stone-grey to deep blue-grey colour, the phenocrysts being olivine (up to 3 mm. long) of various tints from nearly colourless to yellow-green, and melilite (up to 2 mm. square) in thin tabular, less conspicuous crystals which, megascopically, have a shining grey to black appearance. Some of the specimens from near the sources of the flows are internally altered and have become fawn or buff in colour, mottledby white zeolitic specks and reddish brown ferruginous pseudomorphs after olivine and melilite. Some weathered specimens have a brown ochreous crust, but within this the rock is normally fresh.

II.—Radio-activity and the Earth’s Thermal History

Two years ago, in discussing the thermal energy of the earth, I suggested that while it had become impossible to deduce the earths age from its thermal condition alone, Kelvins problem might profitably be reversed by accepting the earths age as a known factor, and deducing with its help the thermal history of the earth. This paper is a first attempt to attack the new problem then suggested. For geological purposes, one of the most fundamental aspects of the problem is that relating to the depth within the earths crust at which temperatures are attained such that, under suitable conditions of pressure, molten rock magmas may exist. The determination of the minimum depth of possible rock fusion is a first essential to any adequate theory of vulcanism, and indeed of igneous activity in general. It is not sufficient, however, to ascertain that depth for present conditions alone, its variation during the earths geological history must also be investigated ; for if, as is generally believed, the earth is a cooling body, the depth must be slowly increasing, and in former periods it must necessarily have been nearer the surface than it is now. In the limiting conditions both of position and time, tlie depth of fusion may have been at, or so near as to be for all practical purposes at, the surface itself. That is to say, at the beginning of geological history the earth may have been in a molten condition at, or immediately below, the then existing surface

XIV.—The Kalsilite-bearing Lavas of Kabirenge and Lyakauli, South-West Uganda

The Western or Toro-Ankole volcanic province of Uganda consists of several areas of tuffs, explosion craters, and rare lava flows situated to the east and south-east of Ruwenzori (text-fig. 1). From north to south these volcanic fields are distinguished under the names ( a ) Rusekere; ( b ) Fort Portal; ( c ) Ndale, formerly known as the “Kyatwa area”; ( d ) Katwe-Kikorongo; ( e ) Bunyaruguru, formerly known as the “Kichwamba area”; and ( f ) Katunga. The Bunyaruguru field extends southwards from Lake George in and adjacent to the north-eastern portion of the Lake Edward section of the western rift. The Kabirenge and Lyakauli lavas (text-fig. 2) occur on the eastern edge of the rift depression immediately north-west of Nyondo crater (E. 30° 04′ 25″; S. 0° 14′ 30″). As a group, they constitute one of the four lava occurrences now known in Bunyaruguru, the other three being those of Chamengo, Mafuru, and Kazimiro (text-fig. 2). Otherwise, lava occurs only as fragments and lumps in the tuffs and as bombs and ejected blocks. The Kabirenge-Lyakauli lavas are also of special interest because, as described later by Professor Holmes, they consist of various members of the mafurite series, characterised by the presence of the recently discovered mineral kalsilite , a polymorph of KAlSiO 4 .

Contributions to the Theory of Magmatic Cycles

It is quite clear that Professor Jolys presentation of his theory of basaltic cycles is, taken alone, far too simple to match the complex details of geological history. In a former paper I attempted to extend the principles he has so persuasively advocated, by adding the conception of a peridotite cycle of longer period, and I was careful to point out that even this extension led to no more than a rough approximation to the actual realities of the earths behaviour. As soon as details are considered, the pursuit of the subject leads its explorer into a maze of difficulties, because of the many interfering factors that must also be taken into account. Nevertheless, a broad survey of the earths history suggests that Joly has surmised what is so far the only kind of process that even begins to correspond with the dominant facts. And that, without any qualification, is a very great achievement.

Age Determination of Carboniferous Basic Rocks of Shropshire and Colonsay

The helium method of determining the ages of fine-grained basic igneous rocks has now been so far developed as to be applicable to various geological and petrological problems, particularly where geological periods are involved, as in the problem here discussed. For details of the history of this development up to the beginning of 1937 reference may be made to Holmes, 1931; Urry, 1933; Lane and Urry, 1935; Urry, 1936 ( b ); Holmes and Paneth, 1936; and Holmes, 1937. During 1937 it was found that many of the helium-ratios on which the ‘helium’ time-scale had been based were too high, because of a previously unsuspected error in radium determination due to reliance having being placed on a radium standard which was seriously at fault. To clear up this embarrassing situation an immediate effort was made by several investigators in collaboration, and the first fruits of their work have recently become available (Evans, Goodman, Keevil, Lane, and Urry, 1939).

A Heteromorph of Venanzite

Venanzite. Venanzite is a volcanic rock consisting essentially of y melilite, leucite, olivine, and a little phlogopitic biotite, which occurs as small lava-flows of late- or post-Pliocene age near San Venanzo, Umbria, Italy. This isolated centre of activity lies north-west of Lake Bolsena and is regarded as an outlying part of the Vulsinian district of the Roman netroaranhic nrovince.

Radioactivity and the Earth's Thermal History. Part V: The Control of Geological History by Radioactivity

In Part IV of this paper the theory that the earth has continuously cooled down from a former molten state was shown to lead to limitations of temperature and to a distribution of rock-types at moderate depths which, taken together, made it impossible for igneous activity to have taken place as we know it to have done. It was argued that this conclusion definitely proved the theory to be wrong. Now, instead of deducing the distribution of rock-types in depth from their radioactive contents, the method of attack will be reversed and an attempt will be made to determine the downward distribution independently, so that the radioactive effects may be deduced without reference to any limiting hypothesis of a steadily cooling earth.