in the Lodes of Dartmoor. 405 



or other artificial excavations, and the names given to them are those 

 by which they are known locally. Their general bearing ranges 

 between E. 25° N. and E. 45° Is''. 



The lodes are simply more or less vertical fissures filled with 

 gangue or veinstone showing no signs of brecciation. In width they 

 vary from that of a mere rift or crack up to 2 feet or more. In 

 many of the open-cuts it will be seen that much of the material 

 worked away is a rotted, somewhat kaolinized granite traversed by 

 innumerable minute vertical fissures crossed by mineralized horizontal 

 pseudo-bedding planes of the granite. In other cases (e.g. Lean's 

 Lodes) massive siliceous veins occupy fissures of considerable width. 



The special interest in these lodes lies in the intimate association of 

 the cassiterite with specular iron-ore, and in this connexion Lean's 

 Lodes, which, by means of an adit level, have been worked for 

 200 fathoms eastwai-ds from the valley, are particularly good 

 examples. 



The veinstone consists of massive quartz with streaks and nests of 

 fine acicular blue tourmaline. 



The tinstone and specular iron-ore occur intimately mingled with 

 one another or as separate veins, and are, in both cases, frequently 

 accompanied by much fine tourmaline. Chalcopyrite and mispickel 

 are absent, and with the exception of a little iron pyrites, which 

 occurs sporadically here and there, sulphides do not occur. 



A chemical examination of the specidar iron-ore, which has the 

 form of small, brilliant spangles, proved it to be quite free from 

 titaniferous minerals. 



The structure of the veins as a whole does not reveal any evidence 

 that there was a definite sequence in the order of deposition of the 

 minerals, which may therefore be regarded as contemporaneous. 

 Fig. 3 is a diagrammatic sketch of a thin slice of the ore as seen 

 through the microscope. The minerals depicted are quartz, cassiterite, 

 specular iron-ore, tourmaline, and a little mica. The cassiterite occurs 

 as granular aggregates or as well-formed prisms, which are often 

 long and slender. The specular iron-ore shows opaque lath-shaped 

 sections, and occurs in close association with the cassiterite, but is 

 never enclosed by the latter. The study of its relationship with the 

 cassiterite indicates that it crystallized after the latter, for it some- 

 times occurs interstitially or appears to have pai'tly crystallized 

 round the cassiterite. It has a closer connexion with the tourmaline, 

 which bears, as a whole, a similar relationship towards the cassiterite. 

 The tourmaline has the form of fine dull-blue needles or fibres 

 radiating from a point or scattered through quartz. It also occurs 

 as zoned prisms with acicular fringes at the ends. The mica is of 

 rare occurrence, and is seen only in minute aggregates, as indicated by 

 the letter Jfin the figure. All these minerals are embedded in quartz 

 of one generation. Under crossed uicols the latter has the appearance 

 of a coarse mosaic. It contains numerous inclusions of slender 

 tourmaline needles and fluid cavities with bubbles. The sequence 

 of crystallization appears to have been (1) cassiterite, (2) specular 

 iron-ore and tourmaline, (3) quartz. 



The actual proportion of cassiterite in the material mined I could 



