First, Every general reader now-a-days is aware that the crust of our globe consists of a series of beds of rock, laid one over the other, like the leaves of a book; and that of these the lowest layers, like the courses of stone in the wall of a building, are the oldest, or were the first laid down. These rocky beds are divided into three groups, of which the lowest, or oldest, is called the primary; the next in order, the secondary; and the uppermost, or newest, the tertiary.
Second, That in certain parts of the world this outer crust of rocks is broken through by living volcanoes, which, with intermissions more or less frequent, belch forth flames and smoke, with occasional torrents of burning lava. That where, or when, the cause of such eruptions is not sufficiently powerful to produce living volcanoes, earthquakes are occasioned; cracks or fissures, more or less wide, are produced in the solid rocks; smoking fumeroles appear; and vapour-exhaling surfaces show that fires, though languid and dormant for the time, still exist beneath. That besides the rocks of lava they have poured out, these volcanic agencies change the surface of a country more widely still by the alterations they gradually effect upon the previously existing slaty, calcareous, or sandstone rocks; converting limestone into marble, and baking sandstone into more or less homogeneous quartz, and common slates or hardened clays into mica slates, gneiss, and granite-like rocks. That such volcanic agencies, producing similar phenomena, have existed in every geological epoch; and though the evidences of these are most extensive and distinct, perhaps, among the rocks of the oldest or primary period, that they are numerous and manifest also among those of the secondary and tertiary periods.
Third, That rocks of every age and kind, when exposed to the action of the air, the vicissitudes of the seasons, the beating of the rains, the force of flowing water, the dash of the inconstant sea, and other natural agencies, crumble down, wear away, or are torn asunder into fragments of every size. These either remain where they are formed, or are carried by winds and moving waters to distances, sometimes very great, but which are dependent on the force of the wind or water which impel them, and on the size or density of the fragments themselves. Thus are our shores daily worn away by the action of the sea, and the fragments distributed along its bottom by the tides and currents; and thus, from the far northern mountains of America, does the Missouri bring down detached fragments thousands of miles into the Gulf of Mexico, whence the Gulf Stream carries them even to the icy Spitzbergen.
Fourth, That over all the solid rocks, almost everywhere is spread a covering of this loose, and, for the most part, drifted matter, consisting of sands, gravels, and clays. These overspread not only valleys and plains, but hill-sides and slopes, and sometimes even mountain-tops, to a greater or less depth. There are comparatively few spots where these loose materials do not cover and conceal the native rocks; but in some localities, and especially in wide plains and deep river valleys, they are sometimes met with in accumulations of enormous depth. In our own island, a depth of two hundred feet of such superficial sands, gravels, and clays, is by no means unusual. They are often sorted into beds alternately coarse and fine, evidently by the action of moving water; and while the great bulk of the fragments of which our English gravels consist can generally be traced to native rocks at no great distance from the spots on which they rest, yet among them are to be found fragments also, which must have been brought from Norway, and other places, many hundred miles distant.
On the surface of these drifted masses we generally live, and from the soils they form we extract by tillage the means of life.
Fifth, That these, occasionally thick, beds of drifted matter—drift we shall for brevity call it—are in some places cut through by existing rivers, the beds of which run between high banks of clay, sand, or gravel, which the action of the stream has gradually worn and washed away. This is seen in many of our own river valleys; and it is especially visible along the great rivers of North America. The effect of this wearing action is to remove, mix up, and redistribute, towards the river’s mouth, the materials which have been scooped out by the cutting water, and thus to produce, on a small scale, along the river’s bed, what had long before been done in the large, when the entire bed of drift through which the river flows was itself spread over the plain or valley by more mighty waters.
These things being understood, a very wide geological examination of gold-bearing localities has shown—
First, That gold rarely occurs in available quantity in any of the stratified rocks, except in those which belong to the primary or oldest group, and in these only when or where they have been, more or less, disturbed or altered by ancient volcanic or volcanic-like action; by the intrusion, for example into cracks and hollows, of veins and masses of serpentine, granite, syenite, and other igneous rocks, in a melted or semi-fluid state.
Second, That among these primary stratified rocks a subdivision, to which the name of Silurian was given by Sir Roderick Murchison, has hitherto, as a whole, proved by far the richest in this kind of mineral wealth; though the slate-rocks below, and the sandstones and limestones above, in favourable circumstances, maybe equally gold-bearing.
Third, That the drifted sands and gravels, in which gold-washing is profitable, occur only in the proximity, more or less near, of such ancient and altered (so called metamorphic) rocks. They are, in fact, the fragments of such rocks broken up, pounded, and borne to their present sites by natural causes, operating long ages ago, but similar in kind to those which now degrade and carry away to lower levels the crumbling particles still torn off from our hardest mountains by the ceaseless tooth of time.