Constructive Action of Groundwater - Some of the mineral material taken in solution by the groundwater is carried in solution to the ocean while part of it is deposited again, sometimes on the surface where the groundwater emerges, and sometimes underneath the surface.
In percolating through the rocks, the water and the carbonic acid gas in the water are under pressure when they take up more carbonate of lime than they can hold in solution under less pressure ; hence, when the water reaches a large cavity or the surface, where the pressure is lowered, some of the acid gas escapes into the air, some of the water is evaporated and part of the mineral matter is deposited.
Veins.
When the mineral matter carried in solution is deposited in cracks or fissures in tlie rocks, it forms veins, in which are formed compounds or ores of different metals such as gold, silver, lead, zinc, copper, etc. Mingled with the ores are variable quantities of other minerals known as gangue or veinstuf consisting of calcite, fluorite, barite, quartz, and other minerals, all of which are carried by the groundwaters into fissures and there deposited to form the vein.
Man is largely dependent upon these veins for the supply of metals needed in the different industries, because in the original condition of the rocks the metals are so scattered and diffused that they cannot be profitably extracted until they are segregated as ores in the veins by the action of the groundwater.
Cave Deposits.
The water that very slowly drips from the roof of a limestone cave is partly evaporated and at the same time permits the escape of part of the carbon dioxide, which causes part of the lime carbonate to be precipitated in the form of an icicle-like deposit called a stalactite. A corresponding projection built up on the floor of the cave is called a stalagmite.
How can you prove that these are carbonate of lime? Many of the stalactites have a small hole running lengthwise through the middle. How do you account for it? If the stalactite and the stalagmite grow together forming a continous deposit from the roof to the floor it is called a column or pillar. A growth along the wall of the cave extending from the floor to the roof is called a pilaster.
In some places this deposition goes on until the cave that was originally formed by the groundwater is completely filled by it. The more massive and compact deposits formed in the cave are quarried and used as onyx marble or Mexican onyx.
Spring Deposits.
The calcite or carbonate of lime is frequently deposited around springs, which are streams of groundwater appearing at the surface. The deposit of the spring is formed similarly to that in the cave, namely, by the escape of the carbonic acid gas and evaporation of some of the water, causing part of the dissolved lime to be deposited. It is frequently deposited on the surface of moss, leaves, or twigs because a large area is there exposed to evaporation. Such porous deposits are called calcareous tufa.
The more massive deposits formed by springs and streams are called traver-tine, a name which is sometimes used for all the deposits of carbonate of lime from solution. The Coliseum and St. Peters and many other large buildings at Rome are constructed of travertine, quarried from an extensive deposit formed hy the springs at Bagni near Rome. Other materials than lime may be brought to the surface by the springs and deposited, such as iron oxide, sulphur, and silica. The silica-depositing springs are generally hot springs.
Induration.
Mineral matter, such as silica, and the carbonates of lime and iron carried in solution by the groundwaters, is sometimes deposited in the open spaces between the grains in a bed of sand or gravel, cementing the particles together and thus changing it into a bed of sandstone or conglomerate. This is one of the principal ways in which beds of sediment are indurated or changed to solid rock.
Frequently the water is brought to the surface by capillarity, where it evaporates, precipitating the mineral matter in the pores. The fact that many sandstones are harder on the surface of the outcrop than in the interior of the bed is accounted for in this way.
In many places in the northern United States, portions of the glacial sand and gravel deposits are cemented by calcite deposited from solution in the groundwater. The student may readily test this by placing a piece of the material in some dilute acid and noting the rapid effervescence, followed by the crumbling of the piece into separate grains or pebbles. It is this small percent of lime that makes the glacial gravels better road-making material than the gravels from the creek beds.
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