ORE DEPOSIT THEORIES. 227 
conclusions. In this paper I will confine myself to a short 
account of the tin occurrences at Ben Lomond.” 
. The granite of the Ben Lomond district is composed prin- 
cipally of quartz and feldspar, with very little biotite. 
Much of the feldspar (orthoclase) occurs in large well-defined 
idiomorphic crystals of the Carlsbad twin type, and these are 
porphyretically distributed throughout the finer-grained 
ground-mass of the granite. At the Mt. Rex Minea large tin- 
stone deposit, some 80 feet in width, occurs. This stone is not 
a typical greissen, asthe mica is only recognisable microscopi- 
cally, but from a genetic point of view the stone isessentially a 
stanniferous greissen. It is evidently ametamorphosed form 
of the surrounding granite, for embedded in the greissen are 
found various pseudomorphs after feldspar, the shape of the 
Carlsbad twin crystal being distinctly recognisable. During 
my visit to the mine in June last year I was able to identify 
the outline of the porphyritic crystals of feldspar in the 
following minerals:—(1) cassiterite, (2) cassiterite and 
quartz, (3) confused muscovite and calcite, (4) chlorite, (5) 
chlorite and quartz, (6) tourmaline (the latter from the 
section north and adjoining the Mt. Rex Mine). As far 
as I am aware, the only other locality where cassiterite has 
been found pseudomorphousafter feldspar is at Wheal Coates, 
near St. Agnes, Cornwall. They were first identified in 
the Mt. Rex deposit by Mr. W. F. Petterd, of Launceston. 
They are not so dense nor so clean cut as some of the speci- 
mens from Cornwall, but they are unmistakably pseudo- 
morphs. The occurrence of pseudomorphs amounts to a defi- 
nite proof that all these minerals may occur as a replacement 
of feldspar. The deposit is traversed by veins of quartz and 
fluorspar, but no topaz has yet been found in the district. 
Quartz veins are abundant both in the granite and the over- 
lying Silurian slates, and the latter may often be shown to 
pass over into veins of pegmatite. In a great many 
instances I found feldspar with the quartz, and specimens 
could readily be collected representing every intermediate 
stage between the pure quartz vein and the typical coarsely 
crystalline quartz-feldspar-mica pegmatites; the latter, how- 
ever, are much less common than the quartz veins of the 
intermediate types. Both the quartz veins and the pegma- 
*The following reports on tin mining districts in Tasmania 
have been issued by the Mines Department of Tasmania since the 
above was written :—W. H. Twelvetrees, F.G.S., Government 
Geologist, Report on the Tin Mines of the Blue Tier, County_of 
Dorset; G. A. Waller, Assistant Government Geologist, The Tin 
Ore Deposits of North Dundas; G. A. Waller, The Tin Ore De 
posits of Mount Heemskirk. - 
