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XIV.—A Contribution to the Petrography of Benguella, based on a Rock Collection made by Professor J. W. Gregory

Published online by Cambridge University Press:  06 July 2012

G. W. Tyrrell
G. W. Tyrrell
Affiliation:
Lecturer in Geology, Glasgow University.
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Extract

Very little is known of the petrography of the Portuguese West African colony of Angola, and of its three provinces that of Benguella is probably least well known in this respect. Granites, gneisses, schists, limestones, and red sandstones have been recognised by the earlier observers of the geology of the region, and detailed descriptions of rocks from Angola, and especially of its northern province, Loanda, have been given by Berg and Holmes. From Benguella itself J. P. Gomes has described the collection of R. P. Leconte, wich contained granites, amphibolite, adinole with epidote veins, various schists, quartzites, diabase-porphyrite, and basic eruptives. Senhor J. P. De Nasciamento in 1912 recorded granite, diorite, basalt, and quartzite on the Benguella plateau.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 51 , Issue 3 , 1917 , pp. 537 - 559
Copyright
Copyright © Royal Society of Edinburgh 1917

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References

page 537 note * Berg, G., “Gesteine von Angola, Saõ Thomé, und St Helena,” Tscher, min. u. petr. Mitth., xxii (1903), pp. 357–362.Google Scholar

page 537 note † Holmes, A., “Contribution to the Petrology of North-western Angola,” Geol. Mag., dec. vi, vol. ii, pp. 228–232; 267–272; 322–328; 366–370, 1915.CrossRefGoogle Scholar

page 537 note ‡ Gomes, J. P., “Échantillons de roches recueillis entre Benguella et Catoco,” Comm. Dir. Trab. Géol. Portugal, vol. iii, fasc. 11, pp. 239–243, 1898.Google Scholar

page 537 note § Nasciamento, J. P. de, and Mattos, A. A. de, A colonisaçãao de Angola, Lisbon, 163 pp., 1912.Google Scholar

page 538 note * The numbers refer to the registration numbers of the rocks in Professor Gregory's collection.

page 539 note * Holland, , “The Charnockite Series,” Mem. Geol. Surv. India, vol. xxviii, part ii, p. 124, 1900.Google Scholar

page 540 note * cf. “Charnockite of the Ivory Coast,” Lacroix, , Comptes Bendus, cl (1910), p. 20.Google Scholar

page 540 note † Holland, op. cit., p. 157.

page 542 note * The terms felsic and mafic (Cross, , Iddings, , Pirsson, , and Washington, , Journ. Geol., xx, 1912, p. 560Google Scholar) are used to denote two main groups of rock-forming minerals, one containing quartz, felspars, and felspathoids, and the other pyroxenee, amphiboles, micas, olivine, iron-ores, etc. Variations in the relative proportions of these groups give rise to the important variations in igneous rocks denoted qualitatively by Brögger's terms, leucocratic and melanocratic. A quantitative meaning may be imparted to the terms felaic and mafic by attaching the prefixes do- and per-, with exactly the same connotation in respect to the mode (actual mineralogical composition) as the analogous terms persalic, dosalic, etc., have in respect to the norm (standard mineral composition) of the American Quantitative Classification.

page 543 note * Lacroix, A., “Sur l'existence à la Côte d'Ivorie d'un séries pétrog. comparable à celle de la charnockite,” Comptes Rendus, cl (1910), pp. 18–22.Google Scholar

page 546 note * Eskola, P., “Petrology of the Orijarvi Region in South-western Finland,” Bull. Comm. Geol. de Finlande, No. 40, 1914, pp. 14, 15, 131.Google Scholar

page 546 note † Holland, op. cit., p. 232.

page 547 note * “Geology and Ore Deposits of Butte, Mont.,”; U.S. Geol. Surv., Prof. Paper No. 74,. 1912, p. 36.Google Scholar

page 549 note * See Iddings, , Igneous Rocks, vol. ii, 1913, p. 104.Google Scholar

page 549 note † Igneous Rocks and their Origin, 1914, pp. 19, 25.

page 552 note * Tscherm. min. petr. Mitth., vol. xxii (1903), p. 359.Google Scholar

page 552 note † Geol. Mag., dec. vi, vol. ii (1915), p. 323.Google Scholar

page 553 note * Gesteinlehre, 3rd ed., 1910, p. 199.

page 553 note ‡ Weed, and Pirsson, , Bull. Geol. Soc. America, vol. vi, 1895, p. 415.Google Scholar

page 553 note § Pirsson, , “Geol. and Petr. of Highwood Mountains, Mont.,” U.S. Geol. Surv., Bull. 237, 1905, p. 104.Google Scholar

page 553 note ∥ Lacroix, Nouv. Arch, du Mus., 1902, p. 179.

page 553 note ¶ Lacroix proposes to use the term theralite in this sense. This, however, ignores Rosenbusch'S plain intention of using this term for the plagioclase-nepheline rocks of plutonic habit. It is true that the original “theralite” of the Crazy Mountains, Montana, on which Rosenbusch erected the group, was erroneously described by Wolff as carrying plagioclase, and is in fact a shonkinite in the sense of Rosenbusch and Iddings. Lacroix'S usage of the term theralite, however, merely perpetuates the original mistake in the diagnosis of the Crazy Mountain rocks. When Rosenbusch discovered the mistake, he promptly withdrew these rocks from the theralite group, and placed them under shonkinite. He then created as his type theralite the unquestionable plagioclase-nepheline rock of Duppau in Bohemia, thus preserving his term theralite with its original connotation.

page 553 note ** Loc. cit., p. 203.

page 553 note † Igneous Rocks, vol. ii, 1913, p. 230.Google Scholar

page 554 note * Mostly soda-orthoclase.

page 554 note † Magnetite and ilmenite.

page 557 note * Rosenbusch, Gesteinlehre, 1910, p. 304.

page 558 note * Rosenbusch, Gesteinlehre, 1910, p. 304.