S4 COLEMAN" ON" THE MICROSCOPIC 



common as inclusions, and also sharp-edged crystals of epidote gleaming like jewels in 

 polarised light. The latter may be secondary, but their complete outline and the want of 

 other evidence of extensive change in the plagioclase, seem to suggest a primary origin. 



Augi/e. — At least three species of augite appear in these rocks. In addition to the 

 common augite we haA'e diallage and a rhombic species, probably hyperstheue. 



The first variety is colorless and transparent, or greyish or green, when moderately 

 fresh. It is apt to take on irregular shapes, filling up the interstices between the crystals 

 of plagioclase. It has the usual rough surface, two not very perfect cleavages inclined 

 abovit 90" to one another, brilliant coloring with polarised light, and an angle of extinc- 

 tion of 35° or 40°. It tends to weather into hornblende, uralite, or greenish chloritic 

 substances, slightly, if at all, pleochroic and magnetite. 



Diallage occurs chiefly as irregularly terminated prisms, with longitutiual fibres or 

 striations. It is iisually dull greenish or brownish grey, only faintly pleochroic, and 

 faintly colored in polarised light. It very commonly encloses regularly oriented scales of 

 a brownish color, sometimes two sets at right angles to one another. Edgewise, these 

 scales look like delicate dashes (Plate II, fig 8). The diallage weathers to the same 

 products as common augite. 



Ht/persfhene, or possibly sometimes bronzite, resembles diallage, but is rhombic, as 

 shewn by the extinction when the striations are parallel to the chief section of either 

 nicol. It should be mentioned, however, that extinction is only faintly marked as a rule, 

 and that some fibres seem to be always light. 



Inclusions similar to those of diallage are found in hypersthene, but generally in 

 much greater numbers, so as even to determine the color. The hypersthene is quite pleo- 

 chroic, yellow and blue in crystals cut so as to show the edges of the inclusions ; red- 

 brown and drab where the flat sides are presented. Patches of hypersthene are often 

 enclosed in diallage crystals. (Plate II, fig. 3). The glints of bronze or copper reflection 

 caught from rocks of this group come from crystals of hypersthene or diallage. 



Hornblende is sometimes to all appearance primary, but generally secondary and 

 uralitic in look. The irregular masses of hornblende have at times a spattered appear- 

 ance, spreading from a centre, and greatly mixed with other minerals. Their color is gen- 

 erally green, but sometimes brown and almost red. Hornblende often forms a margin 

 around the augites, the two minerals having their chief axis in common. (Plate II, figs. 

 1 and 2). 



Biotite occurs in small amounts, and, as a rule, apatite is found in the same speci- 

 mens. Magnetite seems universal, but titanite is rare. Scapolite is found in one or two 

 sections. 



Following Rosenbusch, these rocks may be divided into : — 



Proterobase, containing primary hornblende. 



Diabase Proper, containing only secondary hornblende. 



Gabbro, in a few specimens where diallage replaces common augite entirely. 



Hyperslheii.e Diabase, where much rhombic augite accompanies the monoclinic species. 



It should be stated, however, that the general habitus is very constant, and the varie- 

 ties are united by transitional forms. 



One specimen of diabase proper is worthy of separate mention for its remarkable 

 mode of weathering. The plagioclase, which is in long prisms, has a comparatively clear 



