88 
BASALTIC ROCKS, 
phenomenon, it is found that, in a given district, certain beds gene- 
rally are , and others generally are not productive of metallic trea- 
sures. A particular vein, for example Fryerfold vein in Swaledale, tra- 
verses millstone grits, plates, and cherts ; main limestone ; grits and 
plates ; underset limestone, grits, plates, &c. ; and it is a matter of 
experience that among these beds only the consolidated siliceous and 
calcareous beds are productive ; in the argillaceous plates the veins 
are mostly e dead Also if, in consequence of displacement of beds 
along the vein, solid rocks on one cheek are opposed to argillaceous 
beds, the vein (unless in mining language ‘ very strong’) will probably 
be unproductive and partially contracted; but if solid rocks on one 
side are opposed to the same or to other solid rocks, the vein is in 
a favourable condition for bearing ore. Hence the well known dis- 
tinction of ‘ bearing beds’ and f dead beds’, so important in practical 
mining and geological theory. The sides of the vein being called its 
walls or cheeks, the favourable and unfavourable conditions of a vein 
may be reduced to the following simple formula : 
Cheek A. 
Argillaceous . Vein unproductive and narrow. 
Argillaceous > Vein partially contracted, and poor. 
Gritstone . Vein irregular in its produce. 
Limestone. Vein in the most favourable state. 
Cheek B. 
Argillaceous. 
Gritstone or limestone. 
Gritstone or limestone. 
Limestone. 
In a country like the Dales of Yorkshire, the nature and thickness of 
the * quick’ or bearing beds, extent of them along a vein, and the dip 
of the strata, being well known, the probability of a vein yielding 
good produce at a particular point is reduced to a problem of plane 
geometry ; and every good miner determines accordingly, in what line, 
and at what elevation, to commence his water drift or level. 
It has been attempted from these facts, imperfectly understood, 
to draw a conclusion in favour of a close and necessary dependence 
between the produce of a vein, and the chemical nature of the con- 
taining rocks, so as to permit an inference that the vein itself was 
segregated from the opposite rocks, or else that the whole is of 
