in the Lodes of Dartmoor. 409 



Exmoor and Combe Martin), which were undoubtedly formed after 

 the tin-lodes and were in no way connected with pneumatolytic 

 actions. Such lodes are not here being considered. 



The geological conditions under which specular iron is found in the 

 tin -lodes of Birch Tor are quite in accord with its usual mode of 

 occurrence, since its common habitat is the crystalline schists and the 

 acid eruptive rocks (both volcanic and plutonic), where it is sometimes 

 accompanied by magnetite. It has been artificially prepared by 

 Messrs. Fouque and Levy, and has been observed in copper works 

 slag ^ and as a sublimate in alkali works.'* Its occurrence in the west 

 of England has been frequently recorded, but its connexion with tin- 

 ores has been seen only in small quantities. 



Near Hennock a vein of specular iron-ore 2 feet in thickness has 

 been recorded by Lysons.^ Similar ore has been worked at South 

 Molton, and occurs at Buckfastleigh. During the survey of this 

 district a vein of specular iron-ore in white quartz was found on the 

 north of Water Hill. The specular iron here occurs in unusually 

 large tabular crystals ^ inch across. In small quantities it is present 

 in the lodes of Wheal Maudlin (St. Austell), AVheal Beauchamp 

 (Gwennap), and Tincroft (Camborne). 



To go further afield, it is of interest to know that the mineral 

 occurs" lilentifully in tin-ore veins of the Miickenberg district at 

 Ober-Graupen, and pseudomorphous after wolfram and fluorspar near 

 the Zinnwald, Bohemia. It occurs in Framont (Vosges) near an 

 intrusion of quartz p jrphyry, and has recently been noted in tin-ore 

 of the Booiberg district, Transvaal. 



The general conclusions are that the lodes of Birch Tor may be 

 regarded as a special sub-type of tin veins in which the characteristic 

 minerals are quartz, tourmaline, cassiterite, and specular iron free 

 from sulphides, except a little accessory iron pyrites. In the genetic 

 classification based on mineral associations it occupies a transitional 

 position between haematite lodes and tin-lodes with sulphidic minerals, 

 as follows : — 



1. Haematite (including specular iron-ore. Magnetite and ilmenite 



also come into this group). 



2. Specular iron-ore (with magnetite ?) and cassiterite. 



3. Cassiterite and wolfram, 



4. Cassiterite and wolfram with mispickel, chalcopyrite, and other 



sulphides (and, rarely, argentiferous galena). 

 The veins of types 2, 3, and 4 are further characterized by the 

 presence of such minerals as tourmaline and fluorspar, and are 

 accompanied by alterations of the country rock to greisen, schorl-rock, 

 and china clay rock. 



1 W. Terrill, "Note on Artificial Crystals of Specular Iron found in a Copper 

 Works Slag": Miu. Mag., 1884, vol. v, p. 48. 



- A. Arzruni, "Comparative Observations on Artificial and Natural Minerals": 

 Zeit. f. Kryst.. 1890, vol. xviii, p. 44. 



•* Magna Britannia (Devonshire), 1822, p. cclxviii. 



