"CHIMNEYS" OR "SHOOTS."

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There are contact veins which lie between two kinds of igneous rocks. A dike of diorite may be upheaved in such a position as to form a contact with granite, or with an older dike of diorite or other intruded rock. In places where there are parallel groups of veins, as on the "mother lode," a diorite dike often forms the foot wall of one vein and the hanging wall of another. The black slate of that great mineral belt is in one place the hanging and in another the foot wall, while in many places it forms both walls.

If the gold comes from any of the wall rocks it must be at a great depth, where there is intense heat and great chemical action-at a point where all the metals are much more abundant than near the surface. The nature of the mineral solutions and the metallic vapors filling and passing up through the fissures have more to do with the character of the vein formed than have the wall rocks.

Mineral veins frequently intersect one another. When the intersecting vein fills a fissure in the intersected it shows it to be the more recent, the younger of the two. Some veins are intimately combined at the point of junction, showing them to have been filled at the same. time. As a rule, the work of filling immediately follows the formation of the fissure. A vein intersected by a younger vein is generally rich, as it receives a double charge of mineralized solutions.

Chimneys" or "shoots" of ore in a vein are probably owing to a considerable extent to the character of

156

SUCCESSION OF PINCHES.

the fissure at a greater depth. Though open and roomy near the top the fissure may be narrow or wholly closed at a deeper level, thus permitting the metallic vapors to ascend only at certain points. Thus we see steam rises in columns along the open fissures of hot springs, not in a continuous sheet. Wide places in a vein are more favorable to ore formation than narrow ones. In narrow places the motion of the ascending mineral-bearing solution or vapors is more rapid, therefore not so favorable to the formation of deposits as the wider places. This may cause the apparent "pinching out" of a vein. At such places no sign of the vein will be seen except a seam of clay, but if this is followed it is apt to lead to a broad place in the vein, filled with both quartz and ore. In some veins-owing to the irregular fracture of the rock forming the walls—there are found a succession of such pinches.

It is quite certain that mineral veins have been filled by circulation in the fissures in which they are found, of heated water, aqueous vapors, and various gases, all more or less mineralized. All veins have not been formed in the same way nor by means of vapors and emanations of the same character. No two veins are exactly alike in all respects. Had the veins been filled by means of molten matter from below (as many suppose) their metallic contents would have been the same in all parts, and would have been evenly distributed. There would have been seen no "bonanzas" or "chimneys" of rich ore, with barren spaces between. There can

THE WORK OF AGES.

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be nothing found in or about any lode which shows it to be the result of a quickly completed process. On the contrary, all goes to prove that the formation is the result of a long-continued or periodically repeated process, with modifications at various times of the chemical conditions, degrees of heat and pressure, and variations in the nature of the mineral solutions or metallic vapors. Even the hydrostatic pressure in a column of minerals in solution in a fissure may exert a great influence in the disposition of ore. What might not be affected at a depth of a few hundred or 1,000 feet might be accomplished under the tremendous pressure of 5,000 feet. Doubtless most veins were formed at much greater depth than we now see them. They have become accessible to us through the upheaval and the erosion of what lay above them.

Lodes will more commonly be found in the neighborhood of plutonic rocks-rocks that have solidified beneath the surface-than near volcanic rocks, for the reason that lodes of value could only be formed at a considerable depth under a solid covering. It is useless to look for lodes in sections of a country covered with lava and similar volcanic rocks. Paying mineral veins are much more likely to be found in the older than more recent rocks, whether sedimentary or igneous. They are generally to be found in places where dikes of igneous rocks have been pushed up through the sedimentary rocks, either at the point of contact, between two kinds of rock, or at no great distance on either side.

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WHAT EXPERIENCE SHOWS.

In Cornwall almost the whole of the mineral wealth occurs within a space of two or three miles on each side of a granite and slate contact, but the veins are not richest on the immediate line of contact. In Australia the richest veins are found when the diorite, and other intrusive plutonic rocks, have formed dikes in the stratified rocks. And we see that in California the most noted mines are near dikes of igneous rock. Dikes, not continuous on the surface, may continue underground, some parts being pushed to a greater height than others at the time of their formation. A dike, which is continuous at no great distance below the overlying rock, may appear on the surface as a series of "humps." These may be from half a mile to a mile apart, but from them the prospector will be able to get the course of the dike. Also, where a dike that shows on the surface appears to come to an end, the prospector may take its course and be guided by it in making explorations, in the sections wholly covered by the country rock. The veins lying near the line of the dike will generally prove most valuable.

Usually, when a rich quartz vein has been discovered, there is a "rush" made for "extensions" on the course of the strike of the vein, and at times these locations extend for miles. Let the miner who does not reach the scene of the discovery in time to locate a first extension give no further thought to extensions, but turn his attention to a search for a parallel vein. Systems of parallel veins, more or less regular, depending upon the nature

PARALLELS AND EXTENSIONS.

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of the country rock, are found in almost every quartz mining district of California. The chances for finding a paying parallel vein are often much better than for locating the extension of a newly-discovered lode, and, as has often happened, a parallel vein may be found which will prove richer than the first of the system located.

In California the miner found, when he first turned his attention to the quartz veins, that he was poorly provided with methods and implements for his task, but in the years which have ensued experience and brains have solved the problem of wresting the gold from the rock, and the operation is now performed, in a wellequipped plant, with comparative ease and celerity.

Prior to 1860, quartz mining operations were in an experimental stage, but about that time the great lodes, which were the source from which the rich deposits of the California placer fields came, were discovered, and men began mining them seriously. As a rule then, the miner blasted and picked out his material with crude implements, crushed and pulverized it in a ponderous machine, and extracted the gold by amalgamation on copper plates. This was an operation on the basis of the "free milling" process. But much more than half the gold escaped the seeker in the course of this operation, and only the richest material would pay the expenses of working. Only a small percentage of the gold in the average quartz lode is present in a free state, and for the rest intricate processes must be used