Varieties of Diabase. 73 



terior growths of brown Lasaltic liornl )lende. These two minerals 

 are always in parallel position — i. e.,with their clinopinacoids in 

 common and their extinction directions on the same side of their 

 vertical axes, necessitating, as the writer has suggested before, 

 the change of the plane usually designated the unit orthodome, 

 P^ (101), on hornblende to the basal pinacoid, OP (001), for this 

 mineral. 'i^ 



The parallel growths of pyroxene and hornblende in this rock 

 closely resemble those described and figured l)y Rohrbach in th"e 

 Moravian teschenites.y Tlie rock is in the main quite fresh, but 

 contains considerable patches of serpentinous substance which 

 from their tbrm appear to represent former hypersthene in- 

 dividuals. 



Numbers 45 and 47 are medium-grained non-porphja-itic 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 amygclaloidal diabase whose vesicles 

 are filled with epidote and calcite. 



Among the rocks collected in situ near the foot of the Muir 

 glacier by Mr Gushing are five diabases, three of which are dis- 

 tinguished by the presence of brown hornblende. Of these 

 number 5'' is almost identical with Professor Reid's number 21. 

 Number 9^ 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., 8rd series, vol. xxxix, 1890, p. 356. 

 fUeber die Eruptivgesteine im Gebiete der schlesisch-mahrischen 

 Kreideformation : Tsch. Min.n. Petr. Mitth., vol. 7, 1886, pg. 1. taf. i. 



