Les transactions et les délibérations et rapport de la société philosophique d'Adélaïde, Australie du Sud . ey sont accompagnés d'un rimof la ségrégation curieux de la biotite. Fig. Le schéma 3 montre un tel lieu-régence. Il y a un tuyau aplitiques (a) avec nids de quartz (andtourmaline b) ; c'est entourée par une zone de l'coarsergranite fc) avec moins de la proportion normale de la biotite, 10 et variant en largeur de 1 po jusqu'à 18 in. Cette zone isseparated du granit normal (e) par une étroite bande de terre orrim (d) 1. pour 1J. de large, très riche en biotite. Le long thisbiotitic rim l'érosion différentielle a

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Transactions and proceedings and report of the Philosophical Society of Adelaide, South Australia . ey are accompanied by a curious segregation rimof biotite. Fig. 3 shows diagrammatically such an occur-rence. There is an aplitic pipe (a) with nests of quartz andtourmaline (b); this is surrounded by a zone of the coarsergranite fc) with less than the normal proportion of biotite, 10 and varying in width from 1 in. up to 18 in. This zone isseparated from the normal granite (e) by a narrow band orrim (d) 1 in. to 1J in. wide, very rich in biotite. Along thisbiotitic rim differential erosion has often produced a littledepression or gutter. The small nests of quartz and tourmaline are fairlycommon among the aplites, and appear occasionally in thegranites, and even in the syenite of Rosetta Head.Megascopically one of these nests appears as a dejise mat oftourmaline needles with a little interstitial felspar, the wholesurrounded by a selvage of quartz. Viewed microscopicallythe tourmaline, strongly pleochroic from light brown toindigo blue, is seen to be replacing felspar, the replacement. Fig. 3. starting along cleavage planes, and the whole apparentlyrepresents an alteration , of aplite by magmatic emanations. Of the same general type, though on a larger scale, is apipe about 4 ft. across, the outer rim being of quartz andbiotite with some large felspars, forming a kind of pegmatite.Within this is a zone of quartz, and the centre is occupied bytourmaline. Uralitic Dolerite.—The uralitic diabase described by Dr.Chewings is(?) from a small pipe cutting through the graniteson the eastern side of Green Bay. The outcrop is visible onlyat low tide, and the author was never fortunate enough tofind it uncovered. (7; Vul Prof. Howell in. 11 IV. Petrography. (1) PORPHYRITIC GRANITL. The type specimen was collected on Granite Island. Theauthor is indebted to Mr. Rumbelow, of Encounter Bay, fora specimen from West Island, and this is identical in allrespects with the type. The min

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• bookcentury1800
• bookdecade1870
• booksubjectscience
• bookyear1878