BY LEO A. COTTON. 765^ 



Class iii. — Greisen. 



These deposits occur typically in bunches, though irregular 

 reefs do occur. The rock is highly micaceous, quite a subordinate 

 amount of quartz being present. The mica is of the nature of 

 that mentioned in Class i. 



These greisen bunches are often exceedingly rich in cassiterite, 

 as much as 15cwt. of that mineral to the ton having been ob- 

 tained. The cassiterite is always crystallised, and of a black 

 colour. The masses or reefs pass over, without any abrupt 

 change in composition, into the surrounding granite. The reefs 

 are usually less than one foot in width, and are much more 

 irregular than those of Class i. 'J'he cassiterite may be either 

 coarse (rather larger than a pea) or very fine. I have seen 

 samples carrying more than 10 per cent, of tin, in which none 

 was visible to the naked eye. It must not be supposed, however, 

 that the greisen is always tin-bearing, for much is quite barren. 

 Unlike the greisen of the Altenberg and Zinnwald districts, the 

 Eismore greisen contains no topaz. 



The rock occurs abundantly on the eastern slope of Eismore 

 Hill, where large masses, 6 to 10 feet in diameter, may be seen. 

 So much of the hill is greisenised, indeed, that it is difficult to 

 obtain from this locality a specimen of unaltered granite. Near 

 the Newstead Shaft another development containing very rich 

 tin occurs, but the greisen occurs in a reef of small size in hard 

 acid granite. It is commonly associated with the quartz- 

 quartzose type of vein, occurring in small bunches at the side of 

 the lode. 



Greisen also occurs, but not abundantly, at the Leviathan 

 Mine. It is strikingly absent from the lodes in and near the 

 Tingha Granite. 



Class iv. — Quartz-Greisen. 



A few veins have been observed, consisting chiefly of greisen 

 but containing a central vein of quartz. This quartz is usually of 

 the milk-white, opaque variety, and has been observed to carry 

 crystal-tin. The quartz and greisen are separated by a small 



