Part IX.§2] STOCKS AND STOCK-WORKS. 507 
that some lodes yield tin where they cross granite, and copper where 
they traverse slate; the same lode, as at Botallack, may cross three 
times from the one rock into the other, and each time the same change 
of metallic contents takes place. Some of the lodes which are poor 
in ore in the slate become rich as they cross an elvan (Fig. 309), or, 

Fic. 309.—PuLAN oF EnvaN DYKE (@ 6) TRAVERSED BY A METALLIC VEIN (c e f d), 
WHICH DIES OUT AS IT PASSES INTO THE SURROUNDING SLATE, WHEAL ALFRED, 
GUINEAR (B.). 
on the other hand, the ore is so split up into strings in the elvan, as 
to be much less valuable than in the slate. Similar variations in the 
nature or amount of ores and veinstones with the character of the 
rocks traversed by mineral veins have been generally observed in 
mining districts, even among the most diverse geological formations. 
Decomposition and recomposition in mineral veins.—It has 
been noticed that the “country ” through which mineral veins run 
is often considerably decomposed. In Cornwall this is frequently 
very observable in the granite. Moreover, in most mineral veins 
there occur layers of clay, earth, or other soft friable loamy 
substances to which various mining names are given. In the south- 
west of England the great majority of the remarkable minerals of 
that district occur in those parts of the lodes where such soft earths 
abound. ‘The veins evidently serve as channels for the circulation 
of water both upward and downward, and to this circulation the 
decay of some bands into mere clay or earth, and the recrystallization 
of part of their ingredients into rare or interesting minerals are to be 
ascribed. 
§ 2. Stocks and Stock-works. (Sticke, Stockwerke.) 
‘The cavernous spaces dissolved out in some rocks, more especially 
in limestones and dolomites, may be of any indeterminate shape, and 
may be filled with one or more veinstones or ores, either in sym- 
metrical zones following the outline of walls, floor, and roof, or in 
_ parallel and roughly horizontal bands (Fig. 310). Irregular metalli- 
ferous masses of this kind have long been known in Germany by the 
name of stocks (Stécke) when of large size, smaller aggregations being 
