These large returns may be exaggerations, but they profess to be based on the custom-house books, and may be quite as near the truth as the lower sums of Mr Wyld. But supposing either statement to contain only a tolerable guess at the truth, it may well induce us anxiously to inquire, in the second place, how long is such a supply to continue? _2d_, Two different branches of scientific inquiry must be followed up in order to arrive at anything like a satisfactory answer to this second question. We must investigate both the probable durability of the surface diggings, and the probable occurrence of gold in the native rocks. Now, the duration of profitable gold-washing in a region depends, _first_, on the extent of country over which the gold is spread, and the universality of its diffusion. _Second_, on the minimum proportion of gold in the sands which will pay for washing; and this, again, on the price of labour. The valley of the Sacramento and San Joaquin rivers, in California, is 500 miles long, by an average of 50 miles broad; comprehending an area, therefore, of 25,000 square miles. We do not know as yet over how much of this the gold is distributed; nor whether, after the richest and most accessible spots have been hunted out, and apparently exhausted, the surface of the country generally will admit of being washed over with a profit. We cannot draw a conclusion in reference to this point from any of the statements yet published as to the productiveness of particular spots. But, at the same time, we ought to bear in mind that deserted spots may often be returned to several times, and may yield, to more careful treatment, and more skilful methods in after years, returns of gold not less considerable than those which were obtained by the first adventurers. Besides, if we are to believe Mr Theodore Johnson, “There is no reason to doubt that the whole range of mountains extending from the cascades in Oregon to the Cordilleras in South America, contain greater or less deposits of the precious metals; and it is well known that _Sonora, the northern state of Mexico, is equally rich in gold as the adjoining country of Alta California_. The Mexicans have hitherto proved too feeble to resist the warlike Apaches in that region, consequently its treasure remains comparatively undisturbed.”—(P. 231.) Passing by Mr Johnson’s opinion about the Oregon mountains, what he says of Sénora has probably a foundation in truth, and justifies us in expecting from that region a supply of gold which may make up for any falling off in the produce of the diggings of California for many years to come. The question as to the minimum proportion of gold in the sands of California, or in those of Australia—the state of society, the workmen and the tools, in both countries being much the same—which can be extracted with a profit, or the minimum daily yield which will make it worth extracting, has scarcely as yet become a practical one.