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Tetraethyl lead is a normal constituent of vegetation growing along our highways. Washed grass near Denver contained 3000 ppm (in ash) near major intersections and > 50 ppm for 500 feet downwind. Vegetables grown within 25 feet of a road in upstate New York and western Maryland averaged 80 to 115 ppm.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. International Association for Plant Taxonomy (IAPT) is collaborating with JSTOR to digitize, preserve and extend access to Taxon. SummaryThe use of vegetation in interpreting geological phenomena is becoming an important tool in the search for ore deposits that are buried under thick soil cover or layers of unmineralized rock. Species assemblages and plant density are useful in mapping geologic strata of different chemical composition and reservoir capacity. Hidden ore deposits can be located through chemical analysis of plant tissue, by mapping the distribution of species, and by observing toxic effects caused by an excess of metals as well as signs of faulty nutrition or deranged metabolism in plants whose roots are in contact with ore.Plant indicators of ore deposits may be species that are adapted to living exclusively on rocks or soils that supply unusual amounts of a particular element, or they may be species of wide distribution that favor mineralized ground under certain local conditions because of a change in acidity or availability of major plant constituents. Plants that are not highly tolerant of metals in an ore assemblage may exhibit toxicity symptoms or be completely absent over ore.Geobotanical techniques of mapping indicator plant species and communities, combined with observation of changes in plant appearance can aid the geologist in prospecting for hidden ore deposits.

The geochemist can contribute much information of value toward assessing the effect of environment, including inorganic pollution, on health. The average composition of rocks, soils, plants, and water and also the increments of inorganic substances that can be expected in geologic environments of highmetal content are essential for comparison with metal contents of these components of the environment in areas contaminated by various types of inorganic air and water pollution.Background levels of lead, zinc, nickel, chromium, copper, and manganese in soils and in four classes of vegetation have been estimated from collections that were made in remote areas presumed to be free from inorganic contamination. The trace-metal content of soils and plants varies widely in different geologic provinces of the United States; in those areas of high natural mineralization, additions of metals from man-made pollution may compound a possible hazard. Results of sampling in urban areas show that contamination of vegetation by gasoline lead can be expected for at least 1000 ft back from transportation lanes, and that the lead burden is increasing greatly with time.Ore-treatment plants can also be a source of air contamination for several miles downwind and several thousand feet in other directions. Soils of naturally high metal content in a mining district may be further contaminated with both major and minor elements from smelting operations. Recent studies show that volatile elements are released directly to the air from ore deposits in place. Concentrations of mercury, for instance, may be as much as 20 times background for several hundred feet in altitude over ore deposits in which mercury occurs as a relatively minor constituent. 155