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Societies and Academies

Nature volume 82, pages 328329 (13 January 1910) | Download Citation

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LONDON. Geological Society, December 15, 1909.—Prof. W. J. Sollas, F.R.S., president, in the chair.—R. H. Rastall: The Skiddaw granite and its metamorphism. The visible exposures of the Skiddaw granite are three in number, all very similar; part of the more northerly one is a greisen, which is not here dealt with. The normal granite is more or less porphyritic in structure, with large phenocrysts of perthite, in a coarse or fine-textured ground-mass of orthoclase, plagioclase, biotite, and muscovite. Evidence is brought forward to indicate that the granite is intruded along the axis of an anticline, with a strike approximately E. 15° N. and W. 15° S., the normal direction for the district. The metamorphic aureole is very large, measuring about six miles from east to west, and five miles from north to south. This is out of all proportion to the size of the visible exposures of granite, and it is inferred that the intrusion underlies a large area at a small depth. Within this area three distinct rock-types can be recognised, namely:—(1) black slates; (2) grey flags; (3) grey grits. The metamorphism produced in each of these is described in detail, and it is shown that the commonly accepted, zones of alteration do not hold, since the rocks concerned were originally of very different character. The phenomena displayed may be summed up as an example of a moderate degree of thermal metamorphism; due to the intrusion of a large mass of granite, at a comparatively low temperature, into a series of rocks of variable composition, which had previously undergone dynamic metamorphism.—A. M. Finlayson: The metallogeny of the British Isles. The ore-deposits of the British Isles (tin, copper, lead, zinc, gold) are considered synthetically in their relation to igneous rocks and to tectonics. The great bulk of the deposits of economic importance, including the veins of Cornwall and Devon, the lead, zinc, and copper veins in England, southern Scotland, Wales, and Ireland, are of Hercynian (and Armorican) age. This is shown by the age of the fissuring in many cases (post-Carboniferous to pre-Triassic), by the absence of ore-veins in Jurassic or later formations, and by other evidence. The Tertiary volcanic period was not accompanied by ore-deposition. The ore-deposits are classified according to metallogenetic epochs, and are divided into metallogenetic provinces, as has been done by Prof. L. de Launay with the ore-deposits of Italy, Africa, and Siberia. The essential features of the different groups are summed up. The evidence, collected and sifted, indicates the following zones of ore-deposition:—(1) Pneumatolytic zone: tin, passing up into copper. (2) Deeper vein-zone: copper with gold. Lead and zinc subordinate. (3) Middle and upper vein-zones: lead and zinc. Copper subordinate. The conclusions drawn from the investigation are:—(i) The importance of the physical conditions of the Permo-Trias in favouring ore-deposition in upper zones, (ii) The close connection between metallogenetic and petrographical provinces, and the essential dependence of ore-formation throughout geological time on the differentiation of igneous rocks accompanying great crustal movements. Differences in ore-deposits in different localities and regions appear to be due to primary differentiation of ores accompanying the differentiation of igneous magmas at successive epochs.— F. P. Mennell: The geological structure of southern Rhodesia. The author describes in some detail a portion of what may be termed “the Laurentian area” of Africa. The oldest rocks include all lithological varieties, and exhibit most of the known types of alteration. They comprise a great development of hornblendic rocks (epidiorites and amphibolites); on the other hand, mica-schists, and sheared rocks generally, are conspicuously absent. They include (1) “basement schists”on which the altered sediments were laid down, and (2) altered basic igneous intrusions, simulating rocks of any previous age. All these are older than the granites by which they, and the metamorphic series, are invaded. The vertically bedded “ironstone series”is described, and is compared with similar rocks of the Lake Superior region. They are shown to be especially developed along the eastern border of Matabeleland. The conglomerate beds (or Rhodesian “banket”) are 10,000 feet thick, and rest unconformably upon the ironstone series in the west, both these formations being gold-bearing. The thick crystalline limestones overlying the conglomerate series contain chert and dolomite, the latter rock occurring also as an alteration product from serpentine. Graphite also is found, and is attributed to the insolubility of carbonaceous matter in a highly siliceous magma. The granites occupy the greater part of the area dealt with, and their intrusive character as regards the metamorphic rocks is shown. No fossils are recorded, other than silicified wood, except in the coal-bearing beds, in which occurs Palæomutela keyserlingi of the Russian Permian, as also plants. The paper concludes with a description of the diamond-bearing beds of Rhodesia, which resemble those of Kimberley, and also contain fragments of eclogite.

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https://doi.org/10.1038/082328a0

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