In various parts of the earth’s structure we find solid strata of conglomerate and sandstone, which have been formed at distant and different periods of the world’s history. In other places we may observe the loose materials of the same formation awaiting transformation when exposed to the action of water highly charged with iron, lime, or silica, and we may even see the process taking place at the present day. For example, we will take the broad tracts of level country known in the south of France as the “Landes.” Here we have an excellent illustration of the formation of the gem beds, lacking, however, the precious stones. Below the surface of these plains, and generally at the depth of about three feet, a conglomerate called “allios” has formed, and is still in the process of forming. This stony layer, which is composed of quartz pebbles mixed with sand, has been cemented strongly together by the vegetable and organic matter which has trickled down from the surface of the earth during indefinite periods of time. The action of this organic _débris_ soon renders the conglomerate quite impervious to water, and retards its passing into the sand beds or other strata below. In consequence thereof these broad tracts of level lands become stagnant lagoons of water during the rainy seasons, and all the remains of vegetable and other organic life sink down into these layers of stone, gradually filling up the interstices among the rocks and lesser pebbles. This collection of decayed various organic substances is generally of a gray or blackish color, or may be of a rusty yellow-hue like ferruginous sandstone. Its cementing power has given a special name to the formation it has caused; and to the gold-miner it is known as hard-pan, to the gem-seeker as cascalho or “mellan.” Its peculiar hue, together with the metallic fracture of its layers, has given rise to the belief that it was composed chiefly of a ferruginous oxide. Recent analysis, however, of the “allios” has shown that this idea is partly erroneous, and that the color and the substance of the formation in reality arises from the juice and _débris_ of plants loaded with tannin and other matter. Iron, it seems from these investigations, is present only in small quantities, and also is afforded by the secretion of vegetable life. However, the quantity of iron in the conglomerate varies considerably in different localities, but most of it seems to be acquired from the action of vegetable vitality. These conglomerates of the Landes have been a long time in process of formation; but in other localities, like those of the Cape de Verde, to which the waves of the sea have had access, we may witness the transformation going on with rapidity. No great time is required by nature for this production, but rather the application or conjunction of certain materials exposed to the action of chemical changes and telluric forces.