Below the oldest rocks there lie hidden the sources of that volcanic action which is not yet very correctly understood. Fortunately, we are not now called upon for any explanation of it: it is enough for us that such a force exists; and thrusting below, forces granite and such rocks (which ought to lie quite at the bottom), through a rent made in the upper layers, and still up into the air, until, in some places, they form the summit of considerable mountains. Such changes are not often, if ever, the results of a single, mighty heave, which generates a great catastrophe upon the surface of the earth; they are the products of a force constantly applied through ages in a given manner. In all geologic reasoning we are apt to err grossly when we leave out of our calculation the important element of time. These lower rocks, then--these greenstones, porphyries and granites, sienites and serpentines--thrust themselves in many places through the upper strata of the earth's crust, in such a way as to form mountain ranges. Now, it is a fact, that wherever the oldest of the aqueous deposits--such as those called clay-slates, limestones, and greywacke sandstones--happen to be superficial, so as to be broken through by pressure from below, and intruded upon by the igneous rocks (especially if the said igneous rocks form ranges tending at all from north to south), there gold may be looked for. Gold, it is true, may be found combined with much newer formations; but it is under the peculiar circumstances just now mentioned that gold may be expected to be found in any great and valuable store.