. The Quarterly journal of the Geological Society of London. highly vesicular and amygdaloidal in structure, which alternatewith great masses of ash, lapilli, and scoriaceous fragments. Thevery rugged forms which these rocks assume in weathering causesthem to present a marked contrast with the rocks on which theyrepose. They have evidently been thrust upwards to a certainextent by the great intrusive masses below chem, and are seendipping in both directions from them ; on the west they are inter-sected by the shores of Loch-na-Kael, where their characters canbe conveniently studied. b. Beinn U
. The Quarterly journal of the Geological Society of London. highly vesicular and amygdaloidal in structure, which alternatewith great masses of ash, lapilli, and scoriaceous fragments. Thevery rugged forms which these rocks assume in weathering causesthem to present a marked contrast with the rocks on which theyrepose. They have evidently been thrust upwards to a certainextent by the great intrusive masses below chem, and are seendipping in both directions from them ; on the west they are inter-sected by the shores of Loch-na-Kael, where their characters canbe conveniently studied. b. Beinn Uaig.—The relations of the intrusive granite and felsiteto the overlying felstone lavas is, owing to the inaccessibility of manyparts of the mountain, not conveniently exposed for study upon BeinnGreig. But in Beinn Uaig we have a precisely similar section inwhich the junction in question is beautifully illustrated. This sec-tion is represented in the woodcut, fig. 1, The granite and felsite Fig. 1.—Rocks forming the Summit of Beinn Uaig, Isle of a. Felstone lavas, with agglomerates. b. Syenite-granite graduating into felsite. are seen to give off great veins which traverse the masses of felstonelava and volcanic agglomerate, producing a very sensible degree ofalteration in them along the surfaces of contact; and, further, theseveins are seen to include masses of the traversed rock which havebeen caught up in them. In all these features we recognize thecharacteristics of intrusive granitic masses, which have been so ad-mirably illustrated in the writings of Hutton, Playfair, Webb Sey-mour, and Macculloch. c. Craig Craggen (Section, PI. XXIII. fig. 3).—The valley of thePorsa or Pennygown river exposes on its western side a fine sectionof the mountain called Craig Craggen, which rises to the height of1885 feet. We here find the granites and felsites presenting thesame relations to the overlying felstone lavas as in the two examplesbefore cited. As we trace the lavas u
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
