“The contents of the vein running through the property, which is about 600 feet in length, and crops out on a hill rising about 150 to 200 feet above the level of the Agua Fria Creek, is estimated at about 18,000 tons of ore to the water level only; and how far it may descend below that, is not at present known. “The West Mariposa mine, under Colonel Fremont’s lease, has a vein of quartz which runs the whole length of the allotment, averages six feet in thickness, and has been opened in several places. The assay of Messrs Johnson and Mathey states that a poor specimen of 11 oz. 9 dwt. 18 grains, produced of gold 2 dwt. 17 grains, which would give £1347 per ton; and a rich specimen, weighing 17 oz. 12 dwt. gave 3 oz. 15 dwt. 9 grains, being at the rate of £24,482 per ton.”—(WYLD, pp. 36–39.) The nature and durability of the influence which the discovery and working of these rich veins is likely to have, depends upon their requiring capital, and upon their being in the hands of a limited number of adventurers. In consequence of this they cannot be suddenly exhausted, but may continue to yield a constant supply for an indefinite number of years. In connection with the durability of this supply from the quartz veins—besides the unsettled question as to the actual number and extent of such veins which further exploration will make out—there is the additional question as to how deep these veins will prove rich in gold. Our readers are probably aware that what are called veins are walls, more or less upright, which rise up from an unknown depth through the beds of rock which we have described as overlying each other like the leaves of a book. This wall generally consists of a different material from that of which the rocks themselves consist, and, where a cliff occurs, penetrated by such veins, can readily be distinguished by its colour from the rocks through which it passes. Now, when these veins contain metallic minerals, it has been long observed that, in descending from the surface, the mineral value of the vein undergoes important alterations. Some are rich immediately under the surface of the ground; others do not become so till a considerable depth is reached; while in others, again, the kind of mineral changes altogether as we descend. In Hungary the richest minerals are met with at a depth of eighty or a hundred fathoms. In Transylvania, veins of gold, in descending, become degraded into veins of lead. In Cornwall, some of the copper veins increase in richness the greater the depth to which the mine is carried; while others, which have yielded copper near the surface, have gradually become rich in tin as the depth increased.[20]