Both Cornwall and Devon possess tin mines, which, however, are most important in the county
‘Where England, stretched towards the setting sun,
Narrow and long, o’erlooks the western wave.’—Cowper.
The undulating surface of this arid peninsula, which, being remarkable neither for agricultural nor foreign commerce, has been celebrated since the remotest ages for the mineral riches concealed beneath its barren soil, consists almost exclusively of slaty transition rocks or killas, traversed or intersected by a central granitic range and by dykes of porphyry or elvan which cut the slate and granite, occasionally traversing both in one continuous body of rock, somewhat in the manner of trap-dykes, and evidently of a later formation. The lodes or mineral veins traverse the granite, the slate, and the elvan indiscriminately, but they occur more especially at the junction of granite and slate. They have commonly one prevailing direction, but they invariably throw off into the containing rock ‘shoots, strings, and branches,’ often in such abundance that, instead of one main lode, called a champion lode, the whole is an irregular network of veins. It is not at all certain that the same lode has ever been traced for more than a mile in length. Very often the lode first discovered dwindles to a mere line, whilst some of its offshoots swell out, enlarge, and rival, or even surpass, both in size and richness, the veins from which they have separated.
The metalliferous or valuable contents of a lode generally bear but a small proportion to its unprofitable parts. Instead of forming uniform lines of metal or pure ore, running throughout the whole extent of the vein, they generally occur in what the miners term bunches, or in patches of various sizes and shapes. These very rarely occupy the whole space between the walls or containing sides of the lode, but they are mixed up with a variety of other substances, the chief of which is quartz.
Sometimes a lode is filled with a compact and perfectly solid mass; at other times it abounds in cavities which may occur in any one of the ingredients and also of any size, from those of the hollows of a honeycomb to hollows of several fathoms in length and depth.
In many lodes tin is found associated with copper, and frequently above the latter, so that the upper part of many a copper lode has been worked as a tin lode.
The veins of Cornwall have no determinate size, being sometimes very narrow, and at others exceeding several fathoms in width; sometimes they extend to a great length and depth, at others they end after a short course. They vary so in breadth that in the same lode one part may consist of a mere line between the opposing walls, while another swells to a width of from thirty to forty feet. These great changes, however, seldom happen within several fathoms of each other. Lodes which yield both tin and copper in mixture are considerably larger than those which yield each metal singly. It is also a general fact that the lodes diminish in breadth in proportion to their depth. The richest tin ores are more commonly found between forty and sixty fathoms deep; but in some instances, as in Dolcoath mine, the depth of 200 fathoms has been attained without exhausting the supply, and Tresavean mine has been worked to great profit at more than 320 fathoms from the surface. Tin veins are considered to be good working when only three inches wide, provided the ore be good for its width.
Besides being contained in lodes, tin is also found in alluvial beds, probably resulting from the disintegration of the former during a long series of ages. This stream-tin, as it is called, is met with either in a pulverized sandy state in separate stones, called shodes, or in a continued course of stones, which are sometimes found together in large numbers, and occur at depths varying from one to fifty feet. This course is called a stream, and when rich in ore was formerly called Beauheyl, which is a Cornish word signifying a ‘living stream.’ In the same figurative style, when the stone was but lightly impregnated with tin, it was said to be ‘just alive,’ and dead when it contained no metal.
Tin streams of irregular breadth, though seldom less than a fathom, are often scattered in different quantities over the whole breadths of the moor bottoms or valleys in which they are found. As the confluence of rivers makes a flood, so the meeting of tin-streams makes what is called a rich ‘floor of tin.’ The ore, being thus disseminated both in the alluvium which covers the gentle slopes of the hills, and in that which fills the valleys winding round their base, is easily obtained by conveying over its bed a stream of water, which, by washing away the lighter matter, leaves the heavy ore to be picked up where the operation has been performed.