3. It is by changes of this kind that we become acquainted with the geological structure of the crust of the earth. Mining operations have never extended to a greater depth than three thousand feet, while the inclined position of the strata exposes for examination, along their outcropping edges, _e a c_, the whole series, even to the primary rocks. The upheaval of the granitic rocks, and the removal by denudation of the overlying deposits, shows us the crystalline character which the earthy materials take, when subjected to pressure and cooled from fusion with extreme slowness. Thus we have, exposed to observation, the process of nature in the formation and modification of rocks for several miles in depth. Of the central portions, however, including by far the largest part of the mass of the earth, we have no knowledge whatever. 4. _Springs_, and the other means of obtaining water for domestic purposes, depend in part upon the inclined position of strata, and the broken and uneven condition of the surface, and in part upon the alternation of permeable and impermeable strata. If all the strata were porous, like the sandstones, the water which falls upon the surface would gradually settle through them to the level of the sea; or, if they were all impermeable, like the clays, the water would pass over the surface, and be collected in lakes or the ocean. As it is, the porous structure of the soil and of some rocks acts as a reservoir, from which the water is gradually discharged, and the intervention of impermeable strata prevents its taking a perpendicular direction downwards. Thus, if the stratum _e b_ (Fig. 69) consists of porous rock, and the one below is impermeable, the water which is absorbed at _e_ will appear at _b_ as a spring. Or, if the line _a d_ is a fracture, the water received at c may reappear as a spring at _a_. If the strata were perforated by boring at _e_ till the porous stratum _a_ is reached, the water will rise to the surface, constituting an _Artesian well_. An ordinary well consists of an excavation continued till a stratum is reached which is permanently saturated with water. 5. Most of the _metallic ores_ which occur in the stratified rocks, with the exception of iron, are found in fractures or as dikes. Without these disturbances of the strata, the ores would have remained either sparingly diffused throughout the adjacent strata, or as a part of the melted mass at the volcanic centres. The ores and metals which are found in the primary rocks are accessible only by the bringing up of these rocks to the surface.