IV.—On the Limburgite from near Sasbach

Abstract

So much confusion has existed on the subject of limburgite, that, although it has now been partly dissipated, a record of my own investigations may be of use, at any rate to English petrologists. The rock has been classed by some with the peridotites. From these it is separated by Rosenbusch, who in his latest work places it in a subgroup with the augitites in proximity to the basalts. Zirkel uses limburgite as a synonym for magma-basalt. F. Graeff implies relationship with the nepheline-basalts. Harker's remarks suggest similar conclusions, as do those of Cole, though his opening words are perhaps slightly misleading. All, however, lay so much stress upon the absence of felspar, by giving such definitions as “Limburgite und Augitite sind die felspathfreien Entglieder der Gesteinreihe Trachydolerit, Tephrit,” etc., or “Frei von Feldspar und Felspathoiden ist der Limburgit,” and in some cases by reference to the peridotites, that students may readily overlook the fact that this rock is only free from felspar in the same sense as tachylite or many pitchstones and obsidians.