300 J. A. THOMSON. 



to amygdules, these elliptical plates of quartz may perhaps 

 be more correctly interpreted as corroded plienocrysts 

 round which a secondary deposition of quartz has taken 

 place. The groundmass consists of a fine grained structure- 

 less aggregate of quartz, turbid felspar and magnetite with 

 an abundance of chlorite much stained by limonite. The 

 rock is therefore a porphyry, and perhaps a quartz-porphyry. 

 Streich states that Skirmish Hill is composed in the main 

 of a porphyritic syenite. 



Another rock, certainly a dyke rock, is labelled Cavanagh 

 Range. It is a dark grey, very finely crystalline rock in 

 hand specimens. In section it is seen to be porphyritic, 

 the phenocrysts being in part small euhedral prisms of red- 

 violet, slightly pleochroic titaniferous augite, and in part 

 much larger pseudomorphs of some earlier mineral. The 

 pseudomorphs now consist mainly of chlorite (pennine) with 

 a less amount of carbonates, sphene and flakes of tremolite. 

 Their forms are distinctly suggestive of olivine, although 

 if they represent this mineral, the alteration is an unusual 

 one. The groundmass of the rock is made up largely of 

 small prisms of brown-green hornblende, often green on the 

 margin. They contain occasional small kernels of augite 

 and are surrounded by short fibrous outgrowths of paler 

 hornblende. Next in importance comes felspar in short 

 multiply twinned lath-shaped or radially built forms. The 

 low birefringence, refractive indices less than that of 

 Canada Balsam, and extinction angles up to 15°, refer the 

 species to albite. Here and there large nests of yellow 

 epidote are found, in whose neighbourhood the hornblende 

 is chloritised and carbonates are abundant. Small iron 

 ores are plentifully scattered throughout the groundmass; 

 their form refers them to the magnetite group, while a 

 partial alteration into sphene shows that they are titan- 

 iferous (titanomagnetite). 



