The eruption of these great masses of incandescent rock, impregnated with water and acid gases, through strata of limestone, sandstone, clay, coal, &c., may be expected to produce striking and wonderful chemical changes in the latter. Nor are we disappointed in these anticipations. Whenever we examine the sedimentary materials around volcanic vents, we find that, in contact with the once-fused materials, they everywhere exhibit remarkable evidences of the chemical action to which they have been subjected. The limestones are converted into statuary marble, the sandstones pass into quartzite, the days assume the hardness and lustre of porcelain, while the coals have lost their volatile ingredients and assumed a form like coke or graphite. And these changes are found to extend in many cases to the distance of many hundreds of yards from the planes of junction between the igneous and the sedimentary materials. Among the most interesting effects resulting from the extrusion of masses of incandescent rock, charged with water and various gases, through beds of limestone, clay, sandstone, &c., we may mention the production of those beautiful crystalline minerals which adorn our museums and are so highly prized as gems. By far the larger part of these beautiful minerals have been formed, directly or indirectly, by volcanic agencies. These gems and beautiful minerals are, for the most part, substances of every-day occurrence, which entirely owe their beauty to the crystalline forms they have assumed. The diamond is crystallised carbon, the ruby and sapphire are crystallised alumina, the amethyst and a host of other gems are crystallised silica; and in almost all cases the materials of gems are common and widely diffused, it is only in their finely crystalline condition that they are rare and therefore valuable. [Sidenote: FORMATION OF VOLCANIC MINERALS.]