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 to the width of several feet. The vein at Dalcooth mine, in Cornwall, varies from two or three to forty feet and upwards; and, in some parts, it contracts so as to be little more than six inches across. 8. In Cornwall the first traces of tin and copper are usually found at the surface of the ground, and thence to the depth of 80 or 100 feet beneath; and it is said that no miner has ever yet seen the bottom of a vein, although several have been wrought to the depth of more than 1000 feet. The veins of these metals have, in some instances, been worked to the length of three or four miles. 9. It is frequently observed that metallic veins are separated, from the substances they intersect, by a thin wall, or lining, of minerals different from these substances, and also by a layer of clay on each side of the vein. It is also remarked that the same substance which forms the outer coat of the vein is often intermixed with the ore, or forms layers alternately with it. This has usually the denomination of _matrix_ or _gangue_. 10. There are few mines of any considerable depth that would not be flooded with _water_ from internal springs were not means adopted for drawing off this fluid. The steam engines that are employed for this purpose in some of the Cornish mines are so powerful as to discharge incessantly, both by night and day, a quantity of water, equal to at least 1000 gallons, or near twenty hogsheads, every minute. SYSTEMATIC ARRANGEMENT OF MINERALS. 11. To a superficial observer, perhaps nothing would appear more easy than to describe a mineral. This, however, is by no means the case. The same general appearance sometimes prevails in substances that are very different from each other; and the same stone, in its different states, is often extremely varied in its appearance. To these difficulties it must be added, that the combinations of mineral substances are multiplied to a great extent. A little application, however, particularly if the student be possessed of a collection of arranged and named specimens, which he will have no difficulty in procuring at a reasonable price, will enable him to overcome all the obstacles that otherwise might impede his progress in beginning to acquire a knowledge of this interesting science.[1] 12. The most simple and natural division of minerals is into four classes, of, 1. STONES; 2. SALTS; 3. COMBUSTIBLES; and, 4. METALS; and the following table, which has chiefly been arranged from the system of Werner, the well─known German mineralogist, will exhibit a tolerably correct outline of the classification of these substances. To reduce the whole within the compass of a single page, many of the families, however, have necessarily been omitted. Footnote 1: