MINERAL DEPOSITS. 3 
contain metallic ores, they are styled veins ; but when 
they contain only stony or earthy matters, the miners 
call them dykes. They vary much both in magnitude 
and length. Six thousand feet are considered an un- 
usual length for veins, though, in some instances, veins 
have been traced upwards of four miles. Few veins 
extend more than 1200 feet below the surface of the 
mountains in which they are situated. They are 
usually much inclined ; but they sometimes descend 
in a direction parallel with the beds of rock in which 
they occur. 
5. At the places where dykes or veins pass through 
the earth, they occasionally disjoint the strata in a very 
singular manner (Fig. 4). Some of the coal strata, 
for instance, are thrown down or raised on one side of 
a dyke upwards of a hundred yards ; and the miner, 
after penetrating through this dyke, instead of finding 
the same coal again, meets, on the opposite side, with 
beds of stone or clay. Hence he is frequently at a loss 
how to proceed in searching for the coal of which he 
is in pursuit ; and hence it is that to such dykes the 
peculiar name of faults has sometimes been given. 
6. In England the metallic ores are generally found 
in veins, that form a considerable angle with the re- 
gular strata. This in Cornwall is uniformly the case. 
And it is remarkable, concerning the veins of tin and 
copper of that county, that they run in a direction 
nearly east and west ; whilst the dykes, or veins of 
other substances, run for the most part north and south. 
7. The thickness of veins, and the quantity and 
quality of the ores they contain, differ in every mine. 
Some are only a few inches wide, whilst others extend 
B2 
