350 E. H. Davison — Gastle-an-Dinas Wolfram Mine. 



in insignificant amounts. There is almost complete absence of tin, 

 the ore concentrates containing less than 0"3 per cent. The country 

 rock, on the other hand, is seen under the microscope to contain 

 cassiterite, and samples taken from the slate in the neighbourhood 

 of the granite have been assayed and found to contain as much as 

 1 per cent. These values are, however, too inconsistent to have any 

 economic importance. Another remarkable character is the absence 

 of tourmaline in the lode material, while the country rock is highly 

 tourmalinized. 



Towards the far end of the ujoper level the eastern wall is formed 

 of a granitic rock much decomposed, forming white or pink clay 

 enclosing partly decomposed felspar fragments, quartz, flakes 

 of white mica, and needles of tourmaline. There are also harder 

 bands of greisen composed of quartz, pale brown mica, which 

 contains lithia and is very strongly birefringent, and topaz, with 

 grains and needles of tourmaline. 



In the lower level at the point indicated in the section (Fig. 2) 

 granite appeared in the floor of the lode in the form of a soft clay, 

 containing grains of quartz and flakes of muscovite. This rose 

 slowly towards the south, and passed into a hard fine-grained 

 granite, which extended above the roof of the level. On driving 

 a cross-cut west, the base of the granite Avas seen in the floor where 

 it was underlain by clay-slate, and on continuing the level to the 

 south the slate again appeared above the granite, the surface of 

 the latter continuing with undulations. A winze has been sunk 

 120 yards north of the point where the granite first appears, but 

 though driven to a depth of 87 feet no granite was met with. These 

 facts point to the granite having an irregular tongue-like form rather 

 than to its being a portion of the main mass. 



Development is not sufficiently advanced to show if the lode 

 continues down into the granite, but there are irregular patches of 

 quartz with wolfram in the granite tongues, and it seems probable 

 that the lode fissure was only deflected by the presence of the 

 harder rock. It does not, however, follow that the lode-filling 

 maintains the same character. 



The intimate way in which the granite penetrated the clay-slate 

 can be seen in specimens of clay-slate collected from points in the 

 levels where no granite is in sight ; these are seen to be traversed 

 by fine veins of granitic material which cut across the foliation 

 and vary from an inch or so to less than one-sixteenth of an inch in 

 width. Under the microscope these fine veins are seen to consist of 

 clear quartz, pale brown, highly birefringent mica and tourmaline, 

 with occasional grains of cassiterite. 



The Castle-an-Dinas lode is almost unique in character when 

 compared with other lodes of West Cornwall. It is characterized 

 by the following points : — 



1. The absence of pneumatolytic minerals such as tourmaline, 

 fluorspar, and topaz from the lode contents. 



