VOLCANIC NECKS OF HOBNSBY AND DUNDAS. 501 



basaltic columns. In this basalt the plutonic inclusions 

 have been well preserved. They form large or small peb- 

 bles, rounded, as suggested by Professor David, by magmatic 

 corrosion, and comprise a long series of rocks. They include 

 anorthosites, gabbros, hypersthene gabbros, olivine gabbros, 

 norites, lherzolites, harzburgites, dunites with gabbro- 

 porphyries and olivine dolerite. These present many 

 remarkable and unusual features in their manner of alter- 

 ation, which will be described in some detail. 



The cracks and minor fault planes in the basalt and 

 breccia are filled with chlorite, quartz, calcite or siderite, 

 while well crystallised calcite aragonite barytes and 

 amethystine quartz may be found in small vughs. Pyrites 

 also occurs in irregular grains and well formed crystals. 

 Part II. Penological. 

 In describing the penological features of these rocks it 

 seems best to first consider the mode of occurrence of the 

 minerals of the plutonic inclusions of Dundas and their 

 alterations, following this by an account of their association 

 into various types of rock and their structural and chemical 

 characteristics and relationships. This will be concluded 

 by a description of the other types of rocks studied. 

 Minerals of the Plutonic Inclusions. 

 Felspar. — The only felspars present are plagioclase, 

 sometimes this is labradorite, but often it is bytownite, as 

 shown by the extinction angles, and the refractive indices 

 determined in flakes by the method of Shroeder van der 

 Kolk. It occurs in fairly large grains, almost completely 

 allotriomorphic. Albite twinning is usually shown by broad 

 or narrow bands sometimes crossed by pericline lamellae, 

 which sometimes appear best developed at points of strain.' 



1 Jndd, Quart. Journ. Geol. Soc. 1885, p. 365. W. G. Woolnongh, Proc. 



