VOLCANIC PIPES 347 



crystals showing the usual characters of that mineral, 

 and fairly large well-formed crystals of olivine. It is in 

 a remarkably fresh state for so basic a rock. 1 



The vents and semicircular dyke on the Commonage 

 near Sutherland village are mostly filled with rocks of a 

 thoroughly igneous character ; tuffs, or rocks made up 

 of small fragments of various kinds, including lava and 

 minerals derived from it, are found in three or four of 

 the seven necks, but with them are the igneous rocks ; 

 in the case of three of the pipes the igneous rock is 

 melilite-basalt with more glass and biotite and less 

 augite and melilite than the Spiegel River rock con- 

 tains ; serpentine, calcite and zeolites, the products of 

 alteration of the other constituents, are abundant. 2 The 

 curved dyke is composed of a similar rock. The tuffs 

 in the vents in which the melilite-basalt occurs are light 

 blue sandy rocks containing biotite, ilmenite, serpentine 

 and perofskite in addition to the debris derived from 

 sedimentary beds. The other vents on the Commonage 

 are filled with a dark-coloured amygdaloidal basic glass, 

 and in some cases blocks of sandstone and shale with 

 smaller fragments of the same rocks are imbedded in a 

 matrix evidently composed mainly of altered glass of 

 the nature of the glassy lava in these vents. Serpentine 

 pseudomorphs after olivine are the only large crystalline 

 constituents of this lava, and they appear to have come 

 from another rock, a fine-grained highly altered material 



1 An analysis by Mr. Lewis, one of the Cape Government analysts, is 

 given in G. C., viii., p. 51. A figure of a thin section of the rock will be 

 found in Barker's Petrology, fourth edition, p. 160. 



2 Descriptions of these rocks will be found in G. C., viii, 



