The original source of the metals. It is to the igneous rocks that we may look for the original source of the metals of veins. Lavas contain minute percentages of various metallic compounds, and no doubt this was the case also with the igneous rocks which formed the original earth crust. By the erosion of the igneous rocks the metals have been distributed among sedimentary strata, and even the sea has taken into solution an appreciable amount of gold and other metals, but in this widely diffused condition they are wholly useless to man. The concentration which has made them available is due to the interaction of many agencies. Earth movements fracturing deeply the rocks of the crust, the intrusion of heated masses, the circulation of underground waters, have all cooperated in the concentration of the metals of mineral veins.
While fissure veins are the most important of mineral veins, the latter term is applied also to any water way which has been filled by similar deposits from solution. Thus in soluble rocks, such as limestones, joints enlarged by percolating water are sometimes filled with metalliferous deposits, as, for example, the lead and zinc deposits of the upper Mississippi valley. Even a porous aquifer may be made the seat of mineral deposits, as in the case of some copper-bearing and silver-bearing sandstones of New Mexico.
THE GEOLOGICAL RECORD
What A formation records. We have already learned that each individual body of stratified rock, or formation, constitutes a record of the time when it was laid. The structure and the character of the sediments of each formation tell whether the area was land or sea at the time when they were spread; and if the former, whether the land was river plain, or lake bed, or was covered with wind-blown sands, or by the deposits of an ice sheet. If the sediments are marine, we may know also whether they were laid in shoal water near the shore or in deeper water out at sea, and whether during a period of emergence, or during a period of subsidence when the sea transgressed the land. By the same means each formation records the stage in the cycle of erosion of the land mass from which its sediments were derived. An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion. The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of the earth, and its strata are its leaves. It contains an authentic record of the physical conditions—the geography—of the time and place when and where its sediments were laid.