66 MARIE BENTIVOGLIO. 



the economic importance of tinstone it seemed advisable 

 that it should receive further investigation. 



The specimen of felspathic vein material referred to 

 above, was obtained from the Bark Hut Lode about one and 

 a half miles from Stannum (Vide Plate I). It is rich in tin, 

 which occurs as minute cassiterite crystals, especially 

 abundant in numerous small vughs present, where the 

 crystals line the walls of the cavities. Quartz is also 

 present in well formed prisms contrasting with the decom- 

 posed felspars which form the bulk of the specimen. 



The cassiterite crystals vary in appearance; the majority 

 show highly polished smooth surfaces, while a few have 

 lustreless faces and less smooth surfaces. A special feature 

 of the crystals is their minute size — no intertwinned group 

 observed exceeding 1*5 mm. in any direction, in fact the 

 greater number of groups did not measure 1 mm. in their 

 greatest dimension. 



A number of crystals were carefully loosened from the 

 rock and washed in alcohol, in readiness for examination. 

 Over fifty crystals were selected, mounted, and observed; 

 the majority proved quite valueless for goniometric work, 

 but twelve crystals, in good state of preservation, were 

 finally chosen as being sufficiently well developed to war- 

 rant measuring. In colour the crystals were mostly bluish- 

 black, and generally have a marked lustre, but occasionally 

 the reddish tinge of the so-called "ruby tin" could be 

 observed. 



With a single exception all the crystals are twinned, the 

 twinning occurring on e (101), according to the usual law. 

 Doublets are common, but triplets and fourlings are also 

 frequent. Owing to the mode of attachment, rarely more 

 than half a twin group is developed, thus accounting, 

 probably, for the absence of crystals composed of more 

 than four individuals. 



