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.