Text 16. Vein

Vein, in geology, is a tabular mass of mineral matter, deposited in the fissure, crack, or crevice of a body of rock, and differing in composition from the substance in which it is embedded. Most veins are the result of the gradual precipitation of substances, which were carried by underground waters or gases after the formation of the enclosing material. Veins vary in size from tiny streaks, which may be entirely contained in a small rock specimen, to masses thousands of feet in extent. Veins of quartz and other minerals may also form when magmatic fluids are injected into fissures opened by intrusion of large bodies of igneous rock. In closely spaced stratified layers, this formation is known as a lode.

Many valuable metals and minerals occur in veins of igneous and sedimentary rock. Within a vein the ore may follow certain streaks, known as shoots, or be restricted to pockets of extreme richness. The nonvaluable minerals associated with the ore in a vein are called gangue. A variety of metals are found in veins, usually in chemical combination as minerals, but also, in the case of gold and platinum, in their pure or native form. In California, for example, gold is often found in veins of quartz, the most common gangue mineral.

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NOTES:

• tabular – пластинчатый, плитчатый, плоский, пластовый;

• lode – жила, продуктивный пояс месторождения, жильная толща;

• gangue mineral – жильный минерал.

Text 17. Groundwater

Groundwater is the water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth’s water, or about 20 times more than the total of surface waters on continents and islands.

Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.

The rate of movement of groundwater depends on the type of subsurface rock materials in a given area. Saturated permeable layers capable of providing a usable supply of water are known as aquifers. Typically, they consist of sands, gravels, limestones, or basalts. Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards. Impermeable rocks are known as aquicludes, or basement rocks. In permeable zones, the upper surface of the zone of water saturation is called the water table. When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid a rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.

Although groundwater is less contaminated than surface waters, pollution of this major water supply has become an increasing concern in industrialized nations.

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NOTES:

• aquifer – водоносный слой, пласт или горизонт;

• water table – уровень воды, уровень грунтовых вод.