In a paper read in 1848, before the Royal Institution of London, Sir Roderick Murchison remarks that the principal deposits of gold are found in auriferous “detritus,” and that the same degree of success must not be expected to ensue from exploring the veins, which are ramified through the quartz rock. The result hitherto shown in California fully confirms this theory, as is shown in the following letter from an engineer at St. Francisco, dated 4th April last, after an expedition amongst the localities occupied by the gold diggers:-- “I send you the result of experiments made upon fragments of rock. In each we have operated upon three tons of quartz, reduced to powder, and carefully worked by amalgamation. We have made five experiments upon as many veins in the county of Bath, which is situated between l’Yuba and the River de la Plume. No. 1 has produced [174]3 dollars 53 cents per ton; No. 2, [175]9 dollars 50 cents; Nos. 3 and 4, [176]11 dollars each; and No. 5, [177]17 dollars. “In the county of Nevada, experiments have been made on four different points. The first has given [178]15 dollars per ton; the second, scarcely any gold; the third, [179]14 dollars per ton--this mine, upon which a company had established works, has been abandoned;--and the fourth has given [180]59 dollars, the vein being of an extraordinary richness, and having yielded large returns to the proprietors. “In the county of Eldorado, three different veins did not give a larger return than [181]17 dollars per ton; a fourth equalled the richness of No. 4, in the adjoining county. “In the county of Mariposa, out of eight experiments, three veins gave hardly [182]3 to 7 dollars per ton; three more gave [183]7 to 20 dollars; one gave [184]24 dollars; and one more, [185]38. The two last veins have attracted miners, who are going to work them. No enterprize requires a more careful and a more expensive examination than an auriferous quartz mine. A good vein, yielding [186]36 dollars per ton, may be considered, by moderate people, as worth working; sometimes they are found much richer; but out of all the quartz-crushing mills which have been set up in California, I do not think that one-third are used for mines which are yielding, for any continued period, [187]30 dollars the ton; so that one-half of the works of this nature are suspended.”