Identifier: quarterlyjourna691913geol (find matches)
Title: The Quarterly journal of the Geological Society of London
Year: 1845 (1840s)
Authors: Geological Society of London
Subjects: Geology
Publisher: London (etc.)
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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of impure marble is shown in a specimencollected a few yards farther up the valley. A thin section showson the one hand a hornblende-schist, with rare malacolite, on theother a coarse marble. The intermediate zone, 4 to 5 mm. wide,contains hornblende after malacolite, as also calcite and sphene,the latter in noticeable quantity, although both are greatly sub-ordinate to the other two first-named minerals. In addition thereis some water-clear plagioclase having small extinction-angles.Another instance of a similar association of hornblende-schistand marble was found in the same stream-bed, where a lamina1 inch thick divides two calc-rocks. The former is a hornblende-schist, the hornblende being usually idiomorphic to the extent ofshowing prismatic and clinopinacoid faces, although the latter areoften absent. Microcline and plagioclase, with low extinction-angles, are common, and there is much untwinned felspar andquartz. In regard to accessory minerals, sphene, zircon, and apatite 2n2
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Yol. 69.) METAMORPHOSED SEDIMENTS IN BRITISH EAST AFRICA. 537 are not rare. A few irregular grains of calcite, presenting theappearance of an original mineral, are noteworthy. With a view to the study of the arenaceous sediments, if theymay be so termed, intercalated between the beds of marble, a thinsection has been prepared of a rock which is almost entirely a binarycompound of quartz and felspar. Its thickness was some 18 inches.The calc-rocks between which it was found were of differentfacies: the upper coarse, the lower much finer in grain. The rocksof the section were decomposed, and the section overgrown, but myimpressions in the field were that the gneiss did not represent asill. Most of the felspar is unstriped, but the rock contains alittle acid plagioclase and microcline. Some secondary white mica,derived from felspar, and a few original flakes of a reddish-brownbiotite, also occur. As appears to be usual in these rocks, themicrocline is late in consolidation, and has corro

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