as millstone grit, it was discovered that ore was abundantly distributed in a flat, or streak of ore, between these rocks, the streak being elongated on an E.N.E. direction, that of many of the prevailing fissures, containing lead, in the adjoining country. By following this "flat" downwards, on the dip of the beds, many thousand tons of very excellent sulphuret of lead were obtained in a few years. Subsequently, another mining company sunk a shaft still further upon the dip of the beds, and cut the same kind of deposit in a continuation of the same plane between the millstone grit and carboniferous limestone. From this other streak, or flat of ore, several thousand tons were also raised in a few years. The ground being thus proved for half a mile in length, and pierced by several shafts, a very good illustration is afforded of the extensive occurrence of a metalliferous deposit between two different kinds of rock, and probably also after their deposit, the accumulation of the lead ore being simply in a cavity partially existing between dissimilar beds, instead of in a vertical fissure.* As regards the deposits of mineral matter, including those of the useful metals, in joints of rocks (and the crossing of small veins is sometimes little else than the latter), the partial filling of joints, traversing alike granite, the veins from it, and the schistose rocks through which it has been protruded, may be easily studied at St. Michael's Mount, Cornwall. The joints, well exposed from the insular position of St. Michael's Mount, and from the united action of the sea and atmosphere, give the granite the false appearance of being regularly divided into vertical beds, ranging about E. 10° N., * Warington Smyth, MSS., who further adds, that the underlying limestone is semi-crystalline and grey, abounding in stems of encrinites, and occasionally pierced by "swallow-holes," or water-channels running in various directions, the surfaces of which are smooth except where projecting fossils are found showing their better resistance to the power which removed their once-containing limestone. The roof (millstone grit) is generally a sand, partially calcareous (also containing stems of encrinites), about 18 feet thick, surmounted by a bed of hard sandstone 30 to 36 feet deep; this succeeded by various sandstone and conglomerate beds, amid which there are occasional lenticular masses of limestone. The flat itself was composed of lightcoloured argillaceous matter, from 15 inches to 8 feet in thickness; the metalliferous portion averaging 14 inches thick, but in some places attaining the full height of 8 feet, and consisting entirely of sulphuret of lead. Several streaks of ore were found with the same general direction. The third "flat" found (working in 1849) was often surmounted by 6 or 8 inches of compact carbonate of lead. "The fact," observes Mr. Warington Smyth, "of strings of ore from the main flat' having been traced for 30 feet downwards amid the limestone, and 18 feet into the arenaceous beds above, is sufficient to show that the introduction of the sulphuret of lead has been effected subsequently to the deposit of the millstone grit." As to the loose sandy character of the surmounting bed of rock, this would readily arise either from the decomposition of the millstone grit above, while the lead ore was being deposited, or from subsequent decomposition by the passage of water amid its cementing calcarcous particles. and W. 10° S. A change in the structure of the granite is clearly perceptible towards the joints, and in them are found quartz, mica, topaz, apatite (phosphate of lime), peroxide of tin, wolfram (tungstate of iron), tin pyrites (sulphuret of tin and copper),* schorl, and occasionally other minerals. These are but mineral veins of a particular kind, and on the small scale. As to the peroxide of tin, it is one of the common ores in the fissure veins of the vicinity; and as to wolfram, more of it accompanies the tin ores in certain parts of Cornwall than is convenient for the miner. Quartz is the most abundant mineral in the open spaces, sometimes crystallized, at others filling the joint wholly to its sides. Where these joints traverse the granite veins, and the adjoining (altered) schistose rocks, they sometimes also present interesting examples of differences in their contents, according to the kind of rock forming their walls. The subjoined plan (fig. 289) is one of part of St. Michael's Mount (N.E. side), wherein the granite veins, a, a, a, are seen to traverse the altered slate rocks (which are shaded), a small included portion of the latter being seen in the largest granite vein at c. A joint b, b, traverses both the granite veins and the schistose rock, and d d is a parallel joint less wide. parallel joint less wide. The latter is filled with mica where it crosses the slate, but contains also quartz, and is even occasionally altogether cemented together by that mineral where it traverses the granite veins. The variety of tin pyrites which we thence obtained also contained zinc. The following is an analysis of specimens of this mineral from St. Michael's Mount: The long-celebrated Carglaze tin mine, near St. Austell, Cornwall, also shows joints filled with mineral matter, including peroxide of tin. Many of these have been worked profitably, the granite in which they occur being soft from decomposition. The granite being also white, these joint veins, composed of black schorl and peroxide of tin, mingled with quartz, have a marked appearance, as represented in the annexed sketch (fig. 290). A Fig. 290. large portion of these lines will be found dipping beneath the adjoining slates, as is usual with joint lines bounding the masses of Devonian and Cornish granite, and they are crossed by other joint lines, also in the usual manner. A large proportion of the granite country on the north of St. Austell, particularly in the vicinity of Hensborough, exhibits similar strings, in which schorl and peroxide of tin are intermixed, and so agree with lines, representing joints, that they appear little else also than the filling of spaces among such divisional planes by mineral substances, finally much harder than the granite amid which they were deposited, the latter having become to a considerable extent decomposed. With respect to the schorl in these joint veins, the observer will find, upon studying many of the highly schorlaceous portions of the Devonian and Cornish granites, at their boundaries towards the slates, or surrounding bosses of them, that it may often be found occupying a position near the joints, and, with quartz, sometimes entirely filling up the space between their walls, both minerals appearing to have been derived from the adjacent granite. Not only are certain minerals, including the ores of the useful metals, found in a fissure more frequently adhering to, or accumulated near, particular rocks or modifications of the same rock, in the manner above noticed, but also in some districts, where more * The works upon these small joint veins and upon the fissure veins also traversing the country, the channels, it may have been, through which part, at least, of the contents of the former have been derived, are very extensive in that part of Cornwall, the tin ore having been of excellent quality, and the granite of the district being easily worked, from its state of decomposition. ores than one occur in sufficient abundance to be profitably worked, so that the ground is well explored, fissures in given directions are observed to contain certain of these minerals more than others. Even as regards these also, there would appear to have frequently been conditions under which minerals, chiefly found in fissures taking given directions, were accumulated more in some parts of the same fissure than in others. The mining districts of Devon and Cornwall* may be studied with advantage in this respect, though similar facts are well known in other mining countries in different parts of the world. Referring back to one of those districts (fig. 216, p. 566), it is chiefly in the fissures, v, v, v, having an easterly and westerly direction, that the tin and copper ores are obtained in profitable abundance, while those ranging northerly and southerly, d, d, d, often contain the ores of lead, iron, and some others. There are exceptions, but, as a whole, this distribution of ores is somewhat marked. Upon careful investigation it has been found that the north and south dislocations have been formed subsequently to those having an easterly and westerly direction, the proof being (p. 654), that the contents of the latter have been broken through, as well as the rocks forming their walls, and that new matter has been accumulated in the new fissures. The observer has, therefore, in such cases, not only to bear in mind the direction of the fissures, but also the difference in time when each of the two sets may have been produced, so that if at one time the conditions for the formation of the ores of tin and copper prevailed, and those of other ores at another, the opportunities for the production of various ores in all the fissures were not contemporaneous, but different. This circumstance has to be fully regarded, as well as any influences, causing the deposit of certain substances, which the direction of a fissure itself might occasion.t * The observer will find a considerable mass of important information respecting these mines, in Mr. Henwood's work on the Metalliferous Deposits of Cornwall and Devon; with Appendices on Subterranean Temperature, the Electricity of Rocks and Veins, the Quantities of Water in the Cornish Mines, and Mining Statistics, forming, vol. v. of the "Transactions of the Royal Geological Society of Cornwall," Penzance, 1843. + The study of the different fissures in Cornwall, some containing ores, others not, induced Mr. Carne, in 1822 (Trans. Geol. Society of Cornwall, vol. ii.), to class them under eight divisions, on the principle that the fissures of one epoch had a given direction, and were only cut through by those of subsequent times. By east and west lodes, Mr. Carne says that he means "metalliferous veins whose direction is not more than 30° from those points; by contra-lodes, metalliferous veins whose direction is from 30 to 60° from east and west; and by cross courses, veins whose direction is not more than 30° or 40° from north and south." CHAPTER XXXVI. MODIFICATIONS OF THE CONTENTS OF MINERAL VEINS IN THEIR DEPTH AND TAKING certain minerals for study, and especially the ores of the useful metals, the observer will often find much of interest in the manner in which they may be distributed, as it were, contemporaneously in the same fissure. Certain combinations of rocks will sometimes suggest themselves, if not as the chief cause, at least as among the conditions which may have assisted in rendering the ores of one metal more abundant than those of another in the range of parts of the same mineral vein. At other times this view does not so well accord with the facts observed. The continuation of the great Crinnis lode, running from the coast near Crinnis Island, Cornwall, into the granite, may be noticed as an example of the same fissure being cupriferous amid the slate country, and chiefly stanniferous towards the granite. With respect to the distribution of tin and copper ores in Cornwall, certain of the copper mines in that country are well known to have been worked for tin upon their backs, as the upper parts of mineral veins are often termed in some mining districts, and to have been abandoned when the copper ore was attained beneath, such ores not being considered |