On reflection, it is evident that we could not expect the same results from volcanic eruptions of later ages, since the lava is either vented on dry land, or thrown into deep seas beyond our investigation. The ores or metallic portions of the lava are too minute or limited in amount to make the whole mass rich enough for practical purposes ; and since the mass cools and solidifies on dry land without disintegration, the metals are not separated from the matrix, or precipitated, as they would be if thrown in a molten condition on the water. Beds of ore are, however, frequently formed at the base of trappean formations, or between the strata of volcanic rocks, by gravitation or segregation ; and this form of seams, veins, or beds is frequently met with in volcanic regions. But we may state, as a general fact, that the metallic ores are the direct or indirect results of Plutonic heat, except when stratified with the sedimentary sandstones and slates; and even among these beds a true fissure or vein may occur, as shown in figure 169.
We have attempted to delineate, in this illustration, an ideal section, representing the form of true or fissure veins in their course from the granite through the gneiss to the and even in it. In figure 169, a represents the Paleozoic strata, b the Azoic, and c the granite, d an extinct volcano, and ee the veins.
On issuing from the granite, the vapors and gases seek vents through the stratified overlying Azoic; and since these rocks and slates are generally stratified at high angles, the vent is formed or forced with much more ease between the strata than through or across it. Thus, we frequently find veins of copper and other ores, though the result of sublimation in fissures, lying between the strata in the form of beds. These veins always seek the weakest line of crust or strata, and, of course, escape where the least force is required to find a vent,—whether it be across the strata or between them, or in the line of faults or slips and dikes formed by former convulsions.