. Acta Academiae Aboensis. Science; Mathematics. CRYSTALLINE BASEMENT 41 stage of beginning consolidation in progress, when the pegmatitic dikes cut through the zone in question. These dikes, broad and hght coloured, very markedly visible in the rock ground, are highly differentiated. The border zones are formed of a red feldspar and the midst of the dike is filled v^ith milkwhite quartz. Frequently there is also a smaller exterior border outside the feldspar mass consisting of an evengrained aplite. The limits between the feldspar and the quartz masses are quite distinct. The thickness of the
. Acta Academiae Aboensis. Science; Mathematics. CRYSTALLINE BASEMENT 41 stage of beginning consolidation in progress, when the pegmatitic dikes cut through the zone in question. These dikes, broad and hght coloured, very markedly visible in the rock ground, are highly differentiated. The border zones are formed of a red feldspar and the midst of the dike is filled v^ith milkwhite quartz. Frequently there is also a smaller exterior border outside the feldspar mass consisting of an evengrained aplite. The limits between the feldspar and the quartz masses are quite distinct. The thickness of the dikes varies much and attains several times some meters. It may be noted, that the differentiation appears with the same distinction in dikes of various breadth. The quartz mass contains often well developed crystals of a black tourmaline (schorl). The feldspar is a red ortho- clase and forms, when isola- ted, great individuals (see fig. 6), never mingled with quartz. The habit of these individuals is idiomorphic in some degree, showing partly the pinacoids well developed. Pneu- matolytic minerals are scarcely existent, except the above named tourmaline. Beryl was once found in a pegmatite and was of greater dimensions with the prismatic faces striated (Cerro La Vatea). The flat lying position of the dikes is difficult to explain with regard to the general steep inclination of the schists intruded with granite in this zone. The pegmatitic eruptive material has appa- rently followed independent fissures, opened subsequently to the consolidation of the mingled-rock-zone. The origin of pegmatites has been regarded, as known, partly as. ^ 1 ^ r tr ^ = = S't )^^ r ^ -1 J ) ^ /? r^ ( n FIgf. 6. Great individuals of red potash feldspar lying in a milkwhite quartz mass. The cleavage tracks of the feldspar are visible. Hammer shaft about two decim. Pegmatite dike NE from Villa Please note that these images are extracted from scanned page images that may have been digitally enha
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