Náttúrufræðingurinn - 1955, Page 44

106
NÁTTtJRUFRÆÐINGURINN
Mynd 2. Hluti úr miðkorni með smákomum umhverfis. Stækkun 40 X • — Central
grain surrounded by small sideromelan shards. Magn. 40 X. — Ljósm. T. Tryggva-
son.
fersku gjallgleri er að mestu leyti tvígilt, en þegar glerið binzt vatni,
gengur jámið í samband við meira súrefni og myndar vatnssambönd,
sem eru gul á litinn í þunnsneiðinni.
SUMMARY:
Microscopical Study on Teplira Pisolites
by Tómas Tryggvason.
The pisolites in the tuff stacks S of Jarðbaðshólar mentioned by Prof. Thorar-
insson in a previous paper are studied in thin section. The pisolites are most com-
monly build up by a grain of sideromelan surrounded by one or two layers of
minute sideromelan shards, cemented by an anisotropic medium of low refringence
and indistinct birefringence. In similar way are cemented other occurrences of
basic tuff in Iceland, with which the author is familiar. The pisolites are fringed
by calcite impregnations, and in some cases the hydration of the glass shards has
commenced along the borders on the pisolites. Other tokens of alteration of the
sideromelan are not stated. The impregnation of calcite and the process of hydra-
tion already perceptible in this young tuff (age supposed by Thorarinsson ca. 2500
years) is probably due to hydrothermal influence.