National Geographic : 1892 Mar 21
Varieties of Diabase. terior growths of brown basaltic hornblende. These two minerals are always in parallel position-i. e., with their clinopinacoids in common and their extinction directions on the sane side of their vertical axes, necessitating, as the writer has suggested before, the change of the plane usually designated the unit orthodome, Poo (101), on hornblende to the basal pinacoid, OP (001), for this mineral.* The parallel growths of pyroxene and hornblende in this rock closely resemble those described and figured by Rohrbach in the Moravian teschenites.t The rock is in the main quite fresh, but contains considerable patches of serpentinous substance which from their form appear to represent former hypersthene in dividuals. Numbers 45 and 47 are medium-grained non-porphyritic dia bases whose feldspar forms stout idiomorphic or hypidiomorphic crystals, and whose pyroxene is also to a considerable extent bounded by its own crystal planes. Both are fairly well pre served, although they contain much chlorite, which in number 47 contains highly refractive, spherical bodies. These are iso tropic, but their nature could not be determined. Numbers 20 and 30 are both considerably altered porphyritic diabases, which form transition rocks toward the porphyrites. Their groundmass is an ophitic mass of feldspar and pyroxene, and their phenocrysts mostly, if not altogether, labradorite. Number 33 is a coarsely amygdaloidal diabase whose vesicles are filled with epidote and calcite. Among the rocks collected in situ near the foot of the Muir glacier by Mr Cushing are five diabases, three of which are dis tinguished by the presence of brown hornblende. Of these number 52 is almost identical with Professor Reid's number 21. Number 92 contains large porphyritic crystals of pale pyroxene, which throughout the rock is idiomorphic and seems to have been the earliest product of crystallization. The groundmass is composed of a network of idiomorphic plagioclase laths, con nected by interstitial brown hornblende and serpentinized glass base. This rather unusual sequence of minerals in diabase makes this specimen particularly noteworthy. * Am. Jour. Sci., 3rd series, vol. xxxix, 1890, p. 356. tUeber die Eruptivgesteine im Gebiete der schlesisch-mahrischen Kreideformation: Tsch. Min. u. Petr. Mitth., vol. 7, 1886, pg. 1. taf. i.
1892 May 15
1892 Feb 19