The geology of the Coromandel subdivision, Hauraki, Auckland . head of the Matawai Creek, the altered porphyritic rock provesto be a much-altered dacite or dacite porphyrite. On the eastern side of the main divide dacitic rocks occur in the Whareroa and Omoho Creeks,and much further south a hornblende dacite was identified from Batterv Creek, Mahakirau Valley. Propylitisation of the Volcanic Rocks. A very general alteration of the andesites and dacites is a characteristic feature of the vein-bearing areas of the subdivision. This alteration is best described as propylitisation, the termbeing u
The geology of the Coromandel subdivision, Hauraki, Auckland . head of the Matawai Creek, the altered porphyritic rock provesto be a much-altered dacite or dacite porphyrite. On the eastern side of the main divide dacitic rocks occur in the Whareroa and Omoho Creeks,and much further south a hornblende dacite was identified from Batterv Creek, Mahakirau Valley. Propylitisation of the Volcanic Rocks. A very general alteration of the andesites and dacites is a characteristic feature of the vein-bearing areas of the subdivision. This alteration is best described as propylitisation, the termbeing used to indicate the changes superinduced in andesites and similar rocks by solfataric action,which result in the formation of propylite as defined by Rosenbusch. The highly propylitised rocks, or propylites, have a white, greyish-white, or bluish-white colour,and a rather granular or earthy appearance. Opaque white pseudomorphs of feldspar and traces offerro-magnesian minerals are sometimes observable, the former giving to the rock a rather spotted PLATE Michoicecilitic Hypehsthene Andesite. Tertiart Volcanic Rocks oi Firsq Period, Preeces Point. ( orom indel. Magnification, 33 diameters. Work of Mr. Alexander McKay, Geo. Hull. S. .;.] face p. ?.,. plate xxi.
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Keywords: ., bookcentury1900, bookdecade1900, bookidgeolog, booksubjectgeology
