78 nOKN EXPEDITION — ECOXOMIC GEOLOGY. 



B. — MICA. 



On tlie north .side of Hart Range intrusive dykes of a very coarse-grained 

 granite occur, from a few of which a large quantity of mica has been taken and 

 despatched to Europe via Adelaide. Only a few of the most promising of these 

 dykes have had any attention given to them, and on these very little systematic 

 work has been done. The rea.son for tliis is that, owing to tlie many drawbacks, 

 among which may be mentioned their enormous distance from a railway and want 

 of capital, it has been found necessary to pick out the best of the mica obtainable 

 at or within twenty or thirty feet of tlie surface. As the descent was being made 

 fnjni the sununit of Hart Range, near Mount Brassey, to the valley on the 

 north side, brigiit glistening patches could be seen dotted about, indicating the 

 position of these granite dykes. This glistening effect was produced by the reflec- 

 tion of the sun's rays from the surfaces of innumerable sheets of mica, derived 

 from the breaking up of the large crystals of this mineral, that were liberated as 

 the \\ eathering of the granite jjrogressed. 



The Oolgarna Claim is situated on the western slope of a short northern spur 

 of Hart Kange, and about 300 feet above the valley. A large granite dyke occurs 

 there, worked at the time of my visit by Messrs. Hall and Cluunbers, from which 

 mica was being obtained at and near the surface. Tliis dyke has intruded the 

 gneiss almost vertically, with a N.W. and S.E. strike. Resides the purely surface- 

 work an open cutting twenty-five yards long has been made on the trend of the 

 mass, the height of its eastern face being from fifty to sixty feet. 



In the cutting the distribution (rf the constituents of the granite could be 

 readily perceived. The felspar, of which there are two varieties, is developed on 

 an exceptionally large scale. Some detached pieces must have weighed from one 

 to two cwt. Next in size is the quartz, large, clear, glassy masses of which were 

 seen intimately associated with the felspar, with which it is often intcrcrystallised, 

 even on a large scale. 



The mica, the only mineral commercially important, was found to be largely 

 developed near the walls of the dyke, and to a less extent towards the centre of 

 the mass. That occurring on or near the surface, or near the walls, is usually 

 more or less stained with oxide of iron, and perhaps manganese also; while that 

 obtained from the parts of the rock out of reach of percolating water, i.e., towards 

 tiie centre of the mass, is clear and without stains, though somewhat dark, and 

 generally rather smaller in size. Besides this marketable mica {Muscovite), tliere 

 occur at raie intervals somewhat lai-ge masses of a \ery dark mica (^Biotite), the 



