PETROGRAPHrCAC NOTES. 79- 



have a very scoriaceous appearance. The different structures of the 

 groundmass will be described more in detail further on. 



The colours of the lavas also vary to a great extent. A rich brown,, 

 often with a reddish or purplish tinge, is perhaps the most common, 

 but many are bright red, blue, green or almost black, and a few are 

 white or light grey. The colour appears to be due in most cases to the 

 presence of oxides of iron, but in the green and yellow varieties to the 

 abundance of hornblende or chloritic minerals they contain. The 

 tuffs are generally light coloured, in various shades of pink, buff, and 

 green. 



The specific gravity of the rhyolites varies from 2'42 in the softer 

 altered specimens to about 276 in unaltered varieties. That of some 

 of the tuffs ranges as low as 2-33 and very rarely rises above 2-40. An 

 analysis of the values found for 167 specimens, including tuffs, gives 

 the following results :— 



Specific Gravity. 







Below 2'5 



23 specimens 



or 13-7 per cent 



2'5— 2'55 



32 



19-1 .. 



2-55—2-6 • 



28 



17-0 „ 



2-6 — 2-65 



41 



24-6 „ 



2-65—2-7 



29 



i7 - 3 « 



Above 27 



14 



8-3 „ 



From this it appears that the greater number of the specimens 

 approximate in specific gravity to that of quartz, 2-65. 



The quartz phenocrysts in many cases possess very perfect crys- 

 talline outlines in the form of hexagons and octagons, but more 

 frequently the original shape has been destroyed by corrosion. This 

 frequently results in the production of fantastic forms, inlets, or tongues 

 (Einbuchtungen) of the groundmass projecting far into the interior 

 of the crystal (PI. IX, fig. 1). In some cases the corrosion has taken 

 place more rapidly along zones concentric with the outer boundary of 

 the crystal ; a very perfect example of this is shown in fig. 2, PI. IX. 

 Occasionally the included portions of the groundmass, without visible 



( 79 ) 



