While at San Francisco, an unusual case of success in mining has been made public, and created much excitement even in this city of wonders--so much so as to show that such instances are very rare. Three miners had worked a claim, from which, in the course of a few weeks, they took $84,000. Their expenses for labor, provisions, &c., were about $24,000; But they had with them each about $20,000. I was informed that several hundred miners had been attracted to the same bar by the success of these men, but that no other rich deposits had been found, and, in general, the others were not making a living. Notwithstanding the overgrown fortunes which have been, in some few cases, so rapidly accumulated, I hazard the assertion that in no other part of the United States can there be found so many persons abjectly poor, in proportion to the population, as among those who have resorted to California for purposes of mining. Much is now said, and considerable excitement felt, on the subject of the quartz mining. When two exceptions are made, I know of no locations where the quartz-crushing operations can be at present successfully prosecuted. Two reasons may be given for this opinion. One is, the high price of labor; the second is, the difficulty of replacing parts of the machinery in case of a break. Many individuals and many companies will be losers by entering into the quartz mining speculations. The mode of conducting business in the cities is anomalous. No skill in business transactions; no far-sighted, clear judgment; no long experience in matters of commerce, insure success here. It is much as it is at the mines. A happy hit, if made by the novice--and it is as likely to be made by him as by any--makes the poor man to-day a rich man to-morrow. In the spring of 1849, the single article of saleratus sold for $12 a lb.; it could be purchased in New York at 4 cents. One hundred dollars invested in this single article, deducting all expenses, would yield at the least $25,000. At that same time, building lots in Sacramento City were held at $500; in six weeks they brought $25,000. Let any one calculate for himself what would be the amount made from fifty lots at this rate. In the space of six months, the owner of $100 _might_ be worth a million! Such glittering and gilded castles as these, floating through the imaginations of thousands, led to those wild speculations in lumber, provisions, and other things, which, in the end, have come tumbling down upon the heads of the builders.