830 MR G. W. TYRRELL ON 



with splinters of quartz, and minute wisps of sericite. They probably represent 

 volcanic mud or dust interbedded with the coarser tuffs. 



In a few of the rocks the paste becomes calcareous. Rounded trachyte 

 fragments and mineral chips are embedded in a groundmass consisting of muddy 

 calcite. Scapolitisation of the felspars has taken place in these rocks, but to 

 a much smaller extent than in the non-calcareous tuffs. The volcanic frag- 

 ments may become few and far between, and the rock may then be regarded as 

 a limestone. 



In hand specimens the tuffs are seen to have been affected by an imperfect 

 cleavage which traverses the bedding-planes at a high angle. This, however, seems 

 to have had surprisingly little effect on the internal arrangement of the constituents. 

 In a few specimens the rock-fragments and crystals have a rude alignment in the 

 direction of cleavage. 



The trachyte tuffs are probably the rocks described by Thurach as " diabas- 

 schalstein." They have also been described by Nordenskjold, who has identified 

 the included rock-fragments as " porphyrites." 



III. Igneous Rocks other than Tuffs. 



There are only two specimens of igneous rock in Mr Ferguson's collection, besides 

 the tuffs. One is from a basic sill intruded into the black shales of the lower 

 division of the Cumberland Bay Series ; the other is a porphyry from the moraine at 

 the head of Moraine Fiord. 



The intrusive rock is a dull white in colour, obviously much altered, and pene- 

 trated by veins of quartz. In thin section it is built of large plates of comparatively 

 fresh colourless augite, penetrated by laths of decomposed felspar now only recognis- 

 able by their shapes (Plate XCIV, fig. 6). There is also much interstitial decomposed 

 felspathic material, mixed with pale green to colourless serpentine. The felspars 

 are largely replaced by a dark grey, finely granular, highly refracting material 

 which resembles some varieties of saussurite. The rock may be called ophitic 

 dolerite, ophitic diabase, or ophitic gabbro, according to whether it is desired 

 to place emphasis on the ophitic fabric, the decomposition of the rock, or its 

 coarse texture. 



The porphyry from Moraine Fiord is a pinkish rock containing dull white felspar 

 phenocrysts in a compact aphanitic groundmass. In thin section the phenocrysts are 

 seen to consist of orthoclase, oligoclase-andesine (Ab 5 An 3 ), chloritised hornblende, and 

 biotite, in a fine-grained but holocrystalline quartzose groundmass (Plate XCIV, fig. 5). 

 The felspar phenocrysts form large euhedral crystals, and have suffered considerable 

 sericitisation. The crystals are full of orientated flakes of white mica. The horn- 

 blende has been partially and the biotite completely chloritised. This alteration 

 seems to have disengaged magnetite, which occurs in small specks throughout the 



