. Bulletin of the British Museum (Natural History), Mineralogy. 64 VOLCANIC ROCKS FROM THE CAPE VERDE ISLANDS are slightly higher than in other specimens and the olivine has the highest Fa value observed in the present series of observations (p. 58). [562] is a Stage III basalt from the east chff at Tarrafal, S. Antao. Numerous large pyroxenes and partly serpentinized olivine lie in an almost opaque glassy base containing microlites of labradorite and a b Fig. 15. Olivine-basalts. [a) ,Tarrafal, S. Antao [562]. {b) R. Areia Branca, S. Vicente [587] The last of these dike
. Bulletin of the British Museum (Natural History), Mineralogy. 64 VOLCANIC ROCKS FROM THE CAPE VERDE ISLANDS are slightly higher than in other specimens and the olivine has the highest Fa value observed in the present series of observations (p. 58). [562] is a Stage III basalt from the east chff at Tarrafal, S. Antao. Numerous large pyroxenes and partly serpentinized olivine lie in an almost opaque glassy base containing microlites of labradorite and a b Fig. 15. Olivine-basalts. [a) ,Tarrafal, S. Antao [562]. {b) R. Areia Branca, S. Vicente [587] The last of these dikes which wiU be mentioned is [775], from the same locality as [767, 768]. It is coarsely porphyritic and amygdaloidal and to the naked eye appa- rently rich in olivine, but this appearance is deceptive, as measurement shows that of 58 per cent, phenocrysts only 20 per cent, are of olivine and the rest are of a pale greenish augite. The olivine (Faij) is much altered along cracks to greenish-yeUow serpentine of mean 1535. The augite has a 1-685, 7 i*7o8 (core)-i-7i5 (margin). The groundmass is composed of a greenish-yellow glass crowded with euhedral augites, magnetite, and small poecilitic plates of plagioclase Aueo-sg. No potash feld- spar or feldspathoid has been observed. The irregular amygdales often several millimetres across are filled with calcite and zeolites. (h) Limburgites and Augitites These two feldspar-free types, characterized by phenocrysts of augite and of augite and olivine respectively, set in both cases in a vitreous base containing microlites of augite and magnetite, are widely distributed in most of the islands as dikes and flows amongst the lavas of all stages of activity, and Bebiano (1932: 175 & 180) mentions that much of the area marked as 'basalt' on his map of S. Tiago consists of Recent. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of th
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