ORE DEPOSIT THEORIES. 239 
and greywackes (Silurian), and the gabbros and serpentines, 
which also abound in the district. In this tourmaline por- 
phyry, which often shows evidence of intense alteration, 
especially silicification, veins of tin oxide occur. It is true 
few of these have so far been observed in sitti, but in the 
alluvial workings large blocks of vein-stone are often found 
attached to the porphyry, compésed of nearly pure tin 
oxide. The largest of these blocks, which weighed nearly 
19 cwts., was purchased by the Tasmanian Government, and 
may now be seen in the Museum, Hobart. 
The pyritic-tin deposits (the Cornwall, the Common- 
wealth, the Renison Bell mines, &c.) all occur in the sedi- 
mentary rocks, and have evidently been formed by the 
process of metasomatic replacement. This is best seen in 
the Cornwall Mine, where the rock, which, has undergone 
replacement, is a finely laminated snale in which the various 
layers or laminae have been affecved to a varying extent, 
some of them having been completely replaced, others only 
partly, while others have repelled the attacks of the 
mineralising solutions altogether. The result of this is that 
the whole structure of the original shale is beautifully pre- 
served in the hard pyritic ore bedy. The ore consists of 
iron pyrites and pyrrhotite, with seams of tin oxide, running 
parallel to the planes of stratification; the latter are some- 
times composed of almost pure cassiterite, but in most cases 
the tin is distributed in small grains throughout the pyrites. 
In this dense ore there are none of the minerals present 
which usually accompany tin, but in the centre of the body 
there is a vein about 18 inches in width composed largely 
of the highly characteristic mineral, axinite, with small 
quantities of arsenical pyrites, pyrite, and pyrrhotite. 
This vein cuts across the lines of stratification of the 
country and the bands of ore in the deposit, and certainly 
the filling has at least the appearance of being deposited 
later than the pyritic body. In the Commonwealth Mine, 
there are also veins containing fluorite associated with the 
deposit, as well as axinite. Another remarkable feature of 
these deposits is the presence of a good deal of actinolite. This 
is a metamorphic product, and is caused by the alteration of 
the slates or limestones, as shown by innumerable inter- 
mediate rocks between slate or limestone, with here and 
there a minute needle of actinolite and a rock composed, as 
far as the naked eye can see, of nothing but a dense mass 
of felted actinolite. 
The Renison Bell deposits are also of metasomatic origin, 
but here the rock which has undergone replacement is not 
a finely laminated shale, but an ordinary calcareous slate 
