404 RIDERED ROCK. 



monly, indeed, there is a definite boundary to the mineral masses 

 presented by the rocks on each side, but this is only on the great 

 scale ; and the operations of mining disclose to us innumerable cracks 

 and fissures in these boundary walls, which, when filled 

 by metallic or sparry matters, are called strings, and are 

 frequently worth the labour of following even to great 

 distances from the parent vein, if, indeed, we ought not 

 often to reverse this expression. The notion of miners 

 generally appears to be, that these strings are to be 

 viewed as feeders of the vein, and in proportion to their 

 frequency in many instances is the productiveness of the 

 vein. In the accompanying diagram, the vein is repre- 

 sented as receiving small branches or strings from the 

 neighbouring rock. A rock thus penetrated by strings is 

 sometimes said to be ridered. The rock masses which are 

 Fig. 78. often included in the vein are termed horses ; they are 

 usually well separated from the walls. In many rocks these ridered 

 parts are very greatly altered from their original state. 



It sometimes happens that, in passing through rocks of varying 

 hardness, as limestone, shale, &c., the veins turn flat for a short 

 distance on the hardest and most connected beds (as, for example, 

 on the Tyne bottom limestone of Cumberland), and afterwards 

 continue their ordinary course. These flat parts usually send off 

 strings into the limestone, which may thus be ridered to a consider- 

 able distance. 



Disseminated Veins, &c. Sometimes the mineral is dissemi- 

 nated through the parts of the rock adjoining a vein, or collected in 

 small nests and other closed cavities. This happens not only in the 

 Cornish mines, in killas and granite, but in those in the mountain 

 limestone tracts of the north of England, and even in magnesian 

 limestone. Generally speaking, we may be sure that this metallic 

 impregnation is so related to the veins, that it is an effect of the 

 same agent. Whatever filled the veins, also brought to small 

 distances from them some of their constituent minerals. Certain 

 metals and ores are more liable than others to this lateral diffusion. 

 Native silver, silver glance, red silver ore, native copper, tin ore, iron 

 pyrites, and red iron ochre, are specially noted as occurring in this 

 way ; though copper ore, pyrites, and lead glance seldom exhibit 

 this effect. Galena, however, blende, bitumen, calc spar, and quartz, 

 are found in closed cavities of shells, in mountain limestone, and in 

 other strata. 



Tin Floors, Stockworks, &c. The dissemination of tin ores 

 through some of the rocks of Cornwall was noticed long since by 

 Hawkins, under the title of tin floors. 1 He observed that the whole 

 tenement of Botallack is said to be full of tin floors. At Zinnwald, 

 mineral beds or floors have long been the object of mining adventure. 

 There granite alternates with the tin floors, which consist of quartz 

 and mica, with tin ore, fluor spar, and wolfram. At Breitenbrunn a 

 1 Geol. Soc. of Cornwall Trans., vol. ii. 



