2. Everywhere where stony cavities exist, natural excavations, or apertures opened by man, one observes that which is called stratifications, or superposed beds. The rocks which present this phase are designated stratified rocks. These beds, of a very variable thickness, sometimes of only a few hundred inches, sometimes a hundred yards and more, are distinguished from one another by the color and nature of the substances of which they are composed. Works of art, the boring of wells, the exploding of quarries, and, above all, mines, have given the means of observation to a considerable depth.