BY H. I. JENSEN. 81 



The breccias and tuffs contain an abundance of characteristic 

 boomerang-shaped, bone-and-knuckle, and dumbbell-shaped glass- 

 fragments. 



These pyroclastic rocks were all formed subsequently to the 

 period of extrusion of the comendites and trachytes, and at the 

 commencement of the andesitic and dacitic eruptions. 



El. 15. Loc: Alum Cave, Mt. Flinders. 



Light white to greyish brecciated rock which has bluish stains 

 (smaltite) and bright pinkish-red (erythrite) stains due to cobalt. 

 In handspeciraen it is seen to be made up of fragments of all sizes 

 imbedded in an aphanitic groundmass. 



Under the microscope the fine groundmass may be seen to be 

 composed of fragments of crystals and microlites firmly inter- 

 locked. Nepheline is an abundant constituent, as is also ortho- 

 clase (or anorthoclase). This part of the rock is a tufi: 



On examining the inclusions some are found to consist of tufi' 

 and breccia, others of cryptocrystalline, pseudospherulitic lava. 

 The latter exhibits micropcecilitic fabric; numerous areas which, 

 in plain light, seem to be made up of a dense felt of minute 

 crystallites, extinguish together; the reason obviously being that 

 certain areas which have a felspar-composition but are crowded 

 with minute inclusions, have crystallised out either primarily or 

 by devitrification. As these areas have consolidated under 

 pressure, they do not possess crystalline outlines, but the 

 abundant inclusions present, nevertheless, give them the 

 appearance of being divided up into crystals. 



Felspar is the main component of both tuffy and lava-portions. 

 Next in amount we have the feebly birefringent to isotropic 

 minerals which consist of globulites, crystallites, &c., together 

 with areas of felspathoid. Differentiation between them is rarely 

 possible. Next in abundance we have the nepheline which can 

 be recognised as such. It occurs as hexagonal and prismatic 

 sections showing cleavage at 60°, and characteristic refractive 

 index and double refraction. Next we have a clear colourless or 

 faint bluish mineral with the R.I. of topaz, and a D.R. of about 

 0-030. It occurs associated with decomposing nepheline and 

 6 



