Further Reading

Hunt, John M. Petroleum Geochemistry and Geology. San

Francisco: W. H. Freeman, 1979. North, F. K. Petroleum Geology. Dordrecht, Germany:

Kluwer Academic Publishers, 1986. Seeley, Richard. Elements of Petroleum Geology. New York: Academic Press, 1998.

petrology and petrography Petrology is the branch of geology that attempts to describe and understand the origin, occurrence, structure, and evolution of rocks. Petrography describes the minerals and textures in rock bodies. The two fields are

Meteorites Plate Tectonics

Polarized light micrograph of eucrite, a type of coarse-grained gabbro from an achondritic stony meteorite, recovered from the Frankenstein-range, Hessia, Germany. Magnification: x8 at 6x7 cm size (Alfred Pasieka/Photo Researchers, Inc.)

related but differ in that petrography is largely a descriptive science, whereas petrology uses petro-graphic and other data to deduce the origin and history of rocks.

The petrologic classification of rocks recognizes three main categories with different modes of origin and histories. Igneous rocks crystallized from magma, and include plutonic and volcanic varieties that cooled below and at the surface, respectively. Metamorphic rocks are those that have been changed in some way, such as by the growth of new minerals or structures during heating and pressure from being subjected to tectonic forces. Sedimentary rocks include clastic varieties that represent the broken down, transported, deposited, and cemented fragments of older rocks, as well as chemical and biochemical varieties that represent chemicals that precipitated from a solution.

See also igneous rocks; metamorphism and metamorphic rocks; mineral, mineralogy; sedimentary rock, sedimentation.

Pettijohn, Francis John (1904-1999) American Sedimentologist, Field Geologist Francis Pettijohn was born in Waterford, Wisconsin, on June 20, 1904, and is widely known as the "father of modern sedimentology." Francis became interested in geology while growing up in Bloomington, Indiana, where he often explored the many caves of the region and collected fossils from outcrops near his home. He graduated from high school in Indianapolis in 1921, then entered the University of Minnesota where he received a bachelor of arts in geology in 1924 and a master of arts, also in geology, in 1925. In 1927 he entered graduate school at the University of California at Berkeley, and then transferred to the University of Minnesota, where he received a Ph.D. in Precambrian geology in 1930. He was a professor of geology at Johns Hopkins University in Baltimore from 1952 until his retirement in 1973, and he served as chair of the department there from 1963 to 1968.

Pettijohn is most famous for his studies on the sedimentology and geological evolution of the rocks in the Appalachian Mountains and for the 24 books that he authored or coauthored. Perhaps his most famous book is Sedimentary Rocks, in which the techniques of modern sedimentology were clearly described, and which has been reprinted many times since its first publication in 1949. This book has remained a standard in the field for more than 50 years. Pettijohn published his own autobiography in 1984, a humorous and anecdotal work titled Memoirs of an Unrepentant Field Geologist.

Pettijohn received numerous awards for his work, including the Sorby Medal of the International

Francis Pettijohn

Francis Pettijohn in the field using magnifying lens to examine rock specimen, 1985 (The Ferdinand Hamburger Archives, Johns Hopkins University)

Association of Sedimentologists in 1983, the Twen-hofel Medal from the Society of Economic Paleontologists and Mineralogists, the Wollaston Medal from the Geological Society of London, the Penrose Medal from the Geological Society of America, the Francis J. Pettijohn Medal from the Society for Sedimentary Geology, and an honorary doctor of science degree from the University of Minnesota. He was professionally active, serving as president of the Society of Economic Paleontologists and Mineralogists and Councilor of the Geological Society of America. Pettijohn was elected a member of the National Academy of Sciences and a Fellow of the American Academy of Arts and Sciences.

See also North American geology; sedimentary rock, sedimentation.

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