Larsen, Esper Signius, Jr.

(b. Astoria, Oregon, 14 March 1879; d. Washington, D.C., 8 March 1961),

geology.

The son of a Danish immigrant who had settled in Oregon and had become the first Danish consul in Portland, Larsen attended the local schools. Upon graduation from high school, he did not enter college immediately but worked to ease the financial pressures on the family. Entering the University of California in 1902, Larsen came under the influence of A. C. Lawson and A. S. Eakle while an undergraduate. This led to his taking advanced courses in mathematics and chemistry and contributed to his ultimate decision to make geology and petrology his lifework. After receiving the B.S. degree in 1906, Larsen remained at the university to teach. He left in 1908 but returned to take his doctorate in 1918.

His early studies developed in Larsen the habit of extensive and detailed examination of specimens. He conducted advanced research first as an assistant petrologist in the geophysical laboratory of the Carnegie Institution in Washington, D.C. With H. E. Marwin he developed petrographic techniques, making investigations in optical crystallography and the immersion method of mineral analysis.

In 1909 Larsen was appointed assistant geologist for the U.S. Geological Survey, an association that was one of the most important in his entire professional life. He joined Whitman Cross, who had been studying the volcanic province of the San Juan Mountains of Colorado and New Mexico for fifteen years. Their final report appeared forty–seven Years later, in 1956. In Washington, Larsen was associated with F. E. Wright, who pioneered in the development of optical mineralogy. Their joint paper in 1909 was one of the earliest systematic efforts to establish criteria for geological thermometry.

In 1914 Larsen became a full geologist with the U.S. Geological Survey and served until 1923; for the last five years of this period, he headed the petrology section. Upon leaving the Survey, he became professor of petrography at Harvard University, where he remained until 1949. He then returned to the Survey until failing health forced him to reduce his Survey until failing health forced him to reduce his activities in 1958.

Larsen’s concern with optical mineralogy led him to assemble and tabulate the optical characteristics of more than 600 nonopaque minerals. Microscopy had been applied intensely to the study of rocks and minerals as early as 1850, with Sorby’s development of the thin section technique. It was developed by Zirkel and Rosenbusch in the following years. Larsen extended and refined these early techniques, developing a hollow prism to measure directly the index of refraction of immersion liquids. These methods were refined for the measurment of mineral refraction indexes to three decimal places—until very recently this instrument served as the principal guide to mineral chemistry. Larsen’s systematic methods of petrographic microscopy and his catalog–tables of the optical properties of minerals and the interrelationships of their properties in 1921. A revised handbook written with H. Berman, The Microscopic Determination of the Nonopaque Minerals, appeared in 1934 and remains the single indispensable handbook of optical crystallography.

Larsen’s extensive field studies of the San Juans, the southern California batholith, the Idaho batholith, and later the Highwood Mountains of Montana made him a confirmed magmatist. His extensive microscopic studies of the specimens collected during the annual field seasons of nearly half a century reinforced the conclusions he drew in the field. Larsen strongly supported the laboratory investigations of Minerals under pressures and high temperatures (phase equilibrium studies) associated with T. Vogt and N. L. Bowen.

Larsen’s other major contributions were his development of the concepts of the petrographic province and the variation diagram. He studied the theory of thermal diffusion and applied it to the problem of the cooling of a batholith. He also developed a method of determining the age of igneous rocks using the lead in accessory minerals. His researches in California’s San Diego Country batholithic intrusive rocks had led him to examine the redioactivity of zircon. His retirement from Harvard in 1949 enabled him to devote full time in Washington to applying his methods for determining the age of rocks.

Larsen determined that lead would avoid zircon during the crystallization of magma. Therefore the trace amounts of lead found in zircon were of radiogenic origin, and their quantity was a function of the time since crystallization. He separated zircon from large quantities of igneous rock and proceeded to measure the lead in the zircon spectrographically and to determine the α activity. The formula

is Larsen’s: t = the age of the rock in millions of years; Pb, the lead concentration in parts per million; C= a constant between 2632 and 2013, depending upon the proportion of uranium to thorium; and α the radioactivity in α counts per milligram of the mineral per hour. When a mineral contained unknown amounts of uranium and thorium, C was obtained by the measurements of the total α activity and a fluorescent analysis of the uranium.

In consequence of such contributions, Larsen was widely recognized as the foremost descriptive and theoretical petrologist in America and was awarded highly prized professional medals by both the Mineralogical and the Geological Societies of America. Perhaps his greatest contribution was in training many of the foremost petrologists of the age, not only in the classroom but also in the field and at the Geological Survey offices. His single–minded concentration on geology led to extreme absentmindedness; nonetheless his kindness and the grave consideration that he infallibly extended to those around him made him the object of an affectionate veneration on the part of his students. Among American geologists over the age of forty, the designation “The Professor” is immediately understood to refer to Larsen.

Larsen was survived by his wife of fifty–one years, the former Eva A. Smith, and for less than a year by the second of his two sons, the petrologist Esper S. Larsen III.

BIBLIOGRAPHY

I. Original Works. Larsen’s writings include “Quartz as a Geologic Thermometer,” in American Journal of science, 4th ser., 27 (1909), 421-447, written with F. E. Wright; The Microscopic Determination of the Nonopaque Minerals, Bulletin of the United States Geological Surveyno. 679(1921), 2nd ed., written with Harry Berman, ibid., no.848(1934); “The Igneous Rocks of the Highwood Mountains of Central Montana,” in Transactions of the American Geophysical Union 16th Annual Meeting, pt. 1 (1935), 288-292, written with C. S. Hurlburt et. at.; “Some New Variation Diagrams for Groups of Igenous Rocks,” in Journal of Geology, 46 (1938), 505-520; “Petrographic Province of Central Montana,” in Bulletin of the Geological Society of America, 51 (1940), 887-948; “Geochemistry,” in 50th Anniversary Volume of the Geological Society of America (Washington, D.C., (1941), pp.393-413; “Time Required for the Crystallization of the Great Batholith of Southern and Lower California,” in American Journal of Science, 243A (1945), 399-416; “Batholith and Associated Rocks of Corona, Elsinore, and San Luis Rey Quadrangles, Southern California,” in Memoirs. Geological Society of America, 29 (1948), 1-182; “Method for Determining the Age of Igneous Rocks Using the Accessory Minerals,”in Bulletin of the Geological Society of America, 63 (1952), 1045-1052, written with N. B. Keevil and H. C. Harrison; “Geology and Petrology of the San Juan Region, Southwestern Colorado,” in Professional Papers. United States Geological Survey, no. 258 (1956), 1-303, written with C. W. Cross; “Lead–Alpha Ages of the Mesozoic Batholiths of Western North America,” in Bulletin of the Geological Society of America, no.1070B (1958), 35-62, written with David Gottfried et al.; and “A Reconnaissance of the Idaho Batholith and Comparison With the Southern California Batholith,” ibid., no. 1070A (1958), 1-33, written with R. G. Schmdit.

II. Secondary Literature. See C. S. Hurlburt, Jr., “Esper S. Larsen Jr.,” in American Mineralogist, 47 (Mar. 1962), 450-459; and W. T. Pecora, “Esper Signius Larsen Jr. 1879-1961,” in Bulletin of the Geological Society of America, 73 (Apr.1962), 27-29, followed by a bibliography by Marjorie Hooker, 29-33. See especially the Festschrift volume, “Studies in Petrology and Mineralogy,” American Mineralogist, 35 (1950), 619-958, and the remarkable dedication by J. C. Rabbitt.

Cotland P. Auser