" GREENSTONES " OF WESTERN CORNWALL. 173 



tending in a north-easterly direction from the former locality to the 

 latter, or for a distance of about six and a half miles : their greatest 

 aggregate thickness approaches half a mile ; and all are compre- 

 hended within a belt of less than a mile in width. On either side 

 of these rocks, and in their close proximity, at least two large elvan- 

 courses run parallel with them throughout their extent; and small 

 elvans, or branches from the larger ones, sometimes take their 

 course between the different bands. 



On the Map of the Geological Survey, Camborne is indicated as 

 being the most eastern limit of these rocks ; but it is evident that, 

 although considerably reduced in width, they must be continued at 

 least two miles further east, as they again make their appearance in 

 the workings of South-lioskear, East-Pool, and various other mines. 

 They are generally known as "blue elvans," or "irestones" (iron- 

 stones), the latter name being applied to them by miners on account 

 of their extreme hardness. 



In some places these rocks stand out boldly above the surface of 

 the soil, as a little north of the viaduct situated two miles west of 

 Camborne ; in others they have been laid open cither by railway- 

 cuttings, mining-operations, or quarrying. On account of their 

 hardness and exceptional toughness, they are much used throughout 

 the district as a material for metalling roads, and are occasionally 

 sent to considerable distances for that purpose. Their structure is 

 ordinarily more or less slaty ; but they sometimes break with a 

 hackly or even subconchoidal fracture. In colour they vary from 

 grey to greyish or bluish green. Hocks of this class often enclose 

 specks of pyrites, and are occasionally mottled with bands or veins 

 of a brown or greenish-brown material, which is sometimes imper- 

 fectly crystallized. These veins are probably composed of a mixture 

 of garnet and axinite ; but the crystals are so imperfectly developed 

 in the specimens examined as to render their identification somewhat 

 difficult. 



At No-Man's-Land, two miles east of the village of St. Erth, a 

 quarry has been opened, for road material, upon a band of exceedingly 

 hard bluish-grey rock, which is frequently spotted with pyrites. It 

 is contorted, and somewhat coarse in texture, exhibiting a disposition 

 to break into plates, in accordance with certain parallel planes. 

 This is mixed with a considerable amount of clay-slate, the cleavage 

 of which approximately coincides with the planes of fracture of the 

 enclosing rock ; this slate has evidently been subjected to influences 

 by which its iron has, to a large extent, been converted into hydrated 

 oxide. 



Under the microscope thin sections are found to be composed of 

 filamentary reticulated crystals of various shades of green, very 

 thickly pervading a transparent base, which encloses patches of 

 quartz, magnetite, and pyrites. In addition to the greenish crystal- 

 line mineral before referred to, there is also present a greyish-green 

 nebulous substance, which usually occurs in patches. 



Two miles west of Camborne, after passing through an archway 

 in the embankment supporting the railway viaduct near Carn-Cam- 



