E. H. Davison — Castle-an-Dinas Wolfixtm Mine. 349 



The result of the pneumatolytic alteration is a rock consisting 

 essentially of quartz and tourmaline with some white mica and iron 

 oxide, in which the cleavage, slip-cleavage, and folding of the 

 original slate have been preserved. Under the microscope the 

 slate is seen to be composed of layers of tourmaline needles, 

 alternating with layers of clear quartz, the tourmaline layers con- 

 taining mica and iron oxide. The slate maintains this character 

 close up to the lode walls, though specimens collected from the 

 neighbourhood of the lode fissure were seen to contain fine grains 

 of cassiterite. 



On the summit of the hill the slate is penetrated by a granitic 

 rock, which, as exposed on the hill-top, has the appearanc^e of 

 a granite-porphyry. On tracing the outcrop to the west, down the 

 hill-side, it has a more normal granitic character, but nowhere is it 

 a coarsely crystalline granite such as occurs on Belowda Beacon to 

 the east. The granite outcrop and that exposed in the mine workings 

 seem to be in the nature of apophyses of the Belowda mass to 

 the east. 



On the south of the hill a quartz-porphyry dyke traverses the 

 slate with a strike almost east and west, and a south underlie. 

 The rock is highly tourmalinized, and has been worked for its tin 

 contents. 



The Castle-an-Dinas Lode. 



The lode outcrops on the north side of the hill with a strike of 

 15° E. of N. It is a practically vertical fissure in the slate, and is 

 at present being worked by two adits driven into the hillside at 

 an interval of about 10 fathoms. The width of the lode varies 

 from a few inches to about 3 feet, but an average width of about 

 2 feet is maintained with fair regularity. 



'lit cos ^ 





2"'V.//-, 



Fig. 2. — Section of workings, Castle-an-Dinas Mine. From plan lent by 

 Mr. J. Cbenoweth. 



The lode-filling consists of white vein quartz with irregularly 

 distributed patches and strips of very coarsely crystallized wolfram, 

 the patches having a diameter of a few inches up to 2 feet or so. 

 A remarkable character of the lode-filling is the almost complete 

 absence of minerals other than quartz and wolfram. Occasional 

 copper stains are seen and there is a little native copper but only 



