Petrographische Untersuchungen an Gesteinen des Polzengebietes in Nord-Böhmen

Abstract

THE latest number of the Abhandlungen of the Königl. Sächsischen Gesellschaft der Wissen-schaften contains a memoir by Scheumann on the Tertiary igneous rocks of the Polzen district, in northern Bohemia. These rocks are of the same age as those of the better-known Mittelgebirge, farther west, and have the same alkaline affinities, though there is not the same great preponderance of basic types. In the neighbourhood of Leipa occur numerous volcanic plugs and necks, composed of various alkaline basalts and trachydolerites, with tuffs of corresponding nature. In addition, dykes with a N.E.-S.W. direction are met with throughout the whole district. These have a wider petrographical range, and are discussed at length by the author. The most basic rocks of this series contain 50 per cent, of olivine, with melilite, biotite, haüyne, nepheline, &c. To this type the author gives the name polzenite, but it does not seem to differ essentially from alnöite. From this extreme the rocks range through melilite- and nepheline-basalts, haüyne-basalts, and various trachydolerites to phonolites, the most acid term being a trachytoid phonolite very rich in sanidine. The silica percentage ranges from less than 30 to 58. The whole assemblage of dyke-rocks is regarded as a single series, derived from a common magma by differentiation along definite lines. This conclusion is enforced by chemical analyses, fourteen in number, which yield smooth curves when plotted on a diagram. The author connects the differentiation with progressive crystallisation in the original magma, of trachydoleritic composition; and for a series of rock-types so related he proposes the term pexitropic.

Petrographische Untersuchungen an Gesteinen des Pohengebietes in Nord-Böhmen.

By K. H. Scheumann. Pp. vi + 607-776. (Leipzig: B. G. Teubner, 1913.) Price 8 marks.