A Book of Cornwall

by S. Baring-Gould

Or take it in another way. Cornwall is something like a leg. Let it be a leg vested in a grey stocking. That stocking has so many "potatoes" in it, and each "potato" is eruptive granite.

Granite, however, likewise cracked, formed "faults," as they are called, in parallel lines with the great parent crack to which it owed its appearance, and cracks also formed across these; and through the earlier cracks up gushed later granite in a molten condition, and these are dykes.

Moreover, the satin body not only gave way down its great line of cleavage, but the satin itself in places yielded, revealing, not now the under-linen which boiled out at the great faults, but some material which, I believe, was the lining. So when the granite broke forth there were subsidiary rifts in the slate, and through these rifts a material was extruded, not exactly granite, but like it, called elvan. These elvan dykes vary from a few feet to as many as four hundred in breadth, and many can be traced for several miles. The younger granite intruded into the older granite is also called elvan.

But when the secondary fissures occurred, the intrusive matter was not only a bastard granite, but with it came also tin and copper. And these metallic lines, which run on Dartmoor from E. to W., and in Cornwall from E.N.E. to W.S.W., are called lodes.

The cross-cracks do not contain metal. They are called cross-courses.

In addition there are some capricious veins that do not run in the normal direction, and these are called counter-lodes. Their usual direction is N.E.

The cross-courses, although without metal, are of considerable value to the miner, because, as he knows well, the best lodes are those which are thus traversed.

There is, however, one description of cross-course that is called floocan, and which is packed with clay, and holds back water. These are accordingly not cut through if it can possibly be avoided.

A very curious feature in the lodes is, that after going down to a variable depth the tin is replaced by copper.

Percy was the first to establish this, towards the close of last century. He pointed out that many an old tin mine was in his time worked for copper. And it came to be supposed that this would be found to be an unchanging law: Go deep enough after tin, and you come to copper. But this opinion was shaken when it was found that Dolcoath, the profoundest mine in Cornwall, which had for some time been worked for copper, became next rich in tin. What seems to have been the case was this: when a vent offered, there was a scramble between the two minerals which should get through first and out of the confinement under earth's crust, and now a little tin got ahead; then came copper trampling on its heels, but was itself tripped up by more tin.