Petrography
PETROGRAPHY
PETROGRAPHY deals with the systematic description of rocks. The term is sometimes loosely used as synonymous with "petrology, " which, being the broad science of rocks, is concerned not only with precise description but also with understanding the origin (petrogenesis), modification (metamorphism), and ultimate decay of rocks. Petrography as a science began with a technique invented in 1828 by the Scottish physicist William Nicol for producing polarized light by cutting a crystal of Iceland spar (calcite) into a special prism, which is still known as the Nicol prism. The addition of two such prisms to the ordinary microscope converted the instrument into a polarizing, or petrographic, microscope. By means of transmitted light and Nicol prisms, it became possible to deduce the internal crystallographic character of even very tiny mineral grains, thereby greatly advancing the specific knowledge of a rock's constituents.
But it was a later development that truly laid the foundations of petrography. This was a technique, perfected in the late 1840s by Henry C. Sorby in England and others, whereby a slice of rock was affixed to a microscope slide and then ground so thin that light could be transmitted through mineral grains that otherwise appeared opaque. The position of adjoining grains was not disturbed, thus permitting analysis of rock texture. Thin-section petrography became the standard method of rock study—and since textural details contribute greatly to a knowledge of the sequence of crystallization of the various mineral constituents in a rock, petrography ranges into petrogenesis and thus into petrology.
It was in Europe, principally in Germany, that petrography burgeoned in the last half of the nineteenth century, and American geologists went to Germany for their introduction to this new science. For example, Florence Bascom, the first woman to be hired by the U.S. Geological Survey, studied with Victor Goldschmidt in Heidelberg, Germany, and returned to make major contributions to the nascent study of petrography in America. Her dissertation at Johns Hopkins, completed in 1893, showed that rocks previously thought of as sediments were actually metamorphosed lava flows. This period also coincided with the exploration of the western United States. Notable among the early surveys was the U.S. Geological Exploration of the fortieth parallel under the direction of Clarence King, who became the first director of the U.S. Geological Survey in 1878. The sixth volume of the report of the exploration, Microscopical Petrography (1876), was at King's invitation prepared by Ferdinand Zirkel of Leipzig, at the time acknowledged as one of the two leading petrographers in the world. This publication, in a sense, introduced petrography to the United States.
Subsequent monographs and other publications of the U.S. Geological Survey, as well as some of the state surveys, were replete with beautifully lithographed plates of distinctive rock types as seen in thin sections. Many of these became collector's items, for they are models of scientific accuracy and artistic merit. Many new and interesting rock types were discovered using petrographic methods, and because names of rocks commonly reflect the geography of a type of locality, some exotic names have resulted: for example, shonkinite, from the Shonkin Sag, Montana; ouachitite, from the Ouachita Mountains, Arkansas; and uncompahgrite, from Uncompahgre Peak, Colorado.
Descriptions of rocks are not confined to thin-section studies. One of the earliest members of the U.S. Geological Survey, George F. Becker, recognized that to understand rock minerals properly, it would be necessary to synthesize them from chemically pure components. This awareness led to the establishment of the Geophysical Laboratory of the Carnegie Institution in 1907, of which Arthur L. Day was first director. Chemical principles were applied in investigating sequences and stability ranges of rock minerals, and in parallel studies improved methods of accurate chemical analyses of rocks were developed. Working with colleagues at the Geological Survey, Henry S. Washington, a chemist at the Geophysical
Laboratory, brought out The Quantitative Classification of Igneous Rocks in 1903, a work that had worldwide impact on petrography and petrology. This was followed in 1917 by Chemical Analyses of Igneous Rocks, U.S. Geological Survey Professional Paper 99, perhaps still the largest compendium of chemical analyses of rocks ever brought together. Each analysis, of which there are several thousand, was converted to the author's special classification—a classification still in use, along with the more conventional mineralogical and textural classifications.
Meantime, the physicochemical studies of rock-forming minerals at the laboratory were leading to new interpretations of the origin of rocks, culminating with The Evolution of the Igneous Rocks in 1928 by Norman L. Bowen—a publication that has perhaps had wider influence in petrology than any other emanating from America.
Until the 1920s, when the National Research Council first established a committee on sedimentation, petrographers were concerned chiefly with igneous and metamorphic rocks, for it was these two categories (sometimes grouped as the crystalline rocks) that contained the widest variety of minerals, presented the best-formed crystals, and occurred in the most interesting combinations. Sedimentary rocks, by contrast, appeared relatively uniform and monotonous. Recognition of the economic importance of sediments (especially for their hydrocarbon content) led to an upsurge in sedimentary petrography. Many new authoritative works were published dealing with the special types of petrographic investigations that are appropriate for sediments.
Some of the most exciting developments in petrography involve the sample of moon rocks collected by U.S. astronauts during the first moon mission in 1969. Never before had so many and such highly sophisticated methods of petrographic study been so thoroughly applied: X-ray studies of many kinds, electron microprobes, spectrographic and isotopic analyses, and a host of other advanced techniques, together with the classic petrographic studies. The studies are aided by the fact that since the moon is without atmosphere and apparently without water, moon rocks are not subject to the types of decay that affect most rocks on the earth's surface and tend to obscure thin-section observations and to contaminate (or at least render difficult) chemical studies. Petrographers may well look to the rock formations of the moon and other celestial bodies for new revelations in the field of microscopic petrography.
BIBLIOGRAPHY
Drake, Ellen T., and William M. Jordan, eds. Geologists and Ideas: A History of North American Geology. Boulder, Colo.: Geological Society of America, 1985.
Horowitz, Alan S. Introductory Petrography of Fossils. New York: Springer-Verlag, 1971.
Milner, Henry B. Sedimentary Petrography. New York: MacMillan, 1962.
Stanton, R. L. Ore Petrology. New York: McGraw-Hill, 1972.
Turner, Francis J. Metamorphic Petrology. New York: McGrawHill, 1981.
Williams, Howel, Francis J. Turner, and Charles M. Gilbert. Petrography: An Introduction to the Study of Rocks in Thin Sections. San Francisco: Freeman, 1982.
IanCampbell/a. r.
See alsoGeological Survey, U.S. ; Geology ; Moon Landing ; Paleontology .