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Pirsson, Louis Valentinefree

(03 November 1860–08 December 1919)
  • H. S. Yoder Jr.

Pirsson, Louis Valentine (03 November 1860–08 December 1919), geologist, was born in Fordham, New York, the son of Francis Morris Pirsson and Louise M. Butt. When he was four years old his mother died and he was reared by various relatives. At age nine he was placed on a farm under the tutelage of a strict Presbyterian clergyman who had an extensive personal library. At the age of sixteen he was sent to a boarding school whose headmaster, a strong supporter of Yale, later persuaded him to enter Yale’s Sheffield Scientific School in 1879. When his guardian was no longer able to provide funds, Pirsson became self-supporting. In spite of these difficulties, he graduated with the degree of bachelor of philosophy and honors in analytical chemistry in June 1882. Pirsson continued on at the Sheffield Scientific School as a laboratory assistant and tutor.

The road Pirsson followed from analytical chemist to that of geologist and petrologist was at first a fortuitous route, and when an appealing course was identified, he became highly motivated. On the advice of George J. Brush, director of the Sheffield Scientific School, he accepted an offer to teach at Brooklyn Polytechnic Institute, but the excessive teaching load of elementary classes led to his resignation after a year. Again on the advice of Brush, he accepted a field assistantship to serve under Joseph P. Iddings of the U.S. Geological Survey in 1889 in the Yellowstone National Park. He was so attracted to geology that he returned to New Haven to study under Professor Samuel Penfield, the foremost American mineralogist. After another field season in Montana and receipt of the remainder of his inheritance, he went to Heidelberg, Germany, to study briefly under Carl Heinrich Ferdinand Rosenbusch, the master in mineralogy and petrography. After a short visit in France working under the famous Alfred Lacroix, Ferdinand Fouqué, François E. Mallard, and Auguste Michel-Lévy, he received an offer to return to his alma mater as instructor in mineralogy and lithology at the Sheffield Scientific School in the fall of 1892.

Pirsson first taught mineralogy, then physical geology, and when geology was established as a department in the Sheffield Scientific School in 1893, he initiated a graduate-level course in lithology that attracted many students, presumably because of the opportunities in microscopical petrography. He continued his connection with the U.S. Geological Survey during the summer field seasons in Montana.

Pirsson’s three major contributions evolved from his field studies and teaching experiences: the description of the plutonic alkalic rocks of the Highwood Mountains; a quantitative rock classification for use in classroom generalizations; and an even-handed textbook on physical geology. While mapping the Fort Benton quadrangle in Montana, Walter H. Weed and Pirsson (Geological Society of America Bulletin 6 [1895]: 389–422) discovered an unusual assemblage of alkalic rocks in the form of laccoliths, stocks, and dikes. One highly differentiated igneous mass contained the now classic Shonkin Sag laccolith in the interior of which they found a new rock type they called “shonkinite,” consisting predominantly of an augite and orthoclase. There is no known extrusive or surficial equivalent of this rock such as a lava flow, but on the periphery of the laccolith is another new rock type they called “missourite” that is the plutonic or deep-seated equivalent of leucite basalt. (Pirsson is among the top ten investigators generating new rock names.) Many other interesting petrologic features, including alkaline rock types and intrusive forms described by them from the Highwood Mountains, have attracted worldwide attention and additional studies by others. Pirsson summarized his extensive studies of the Highwood Mountain geology and petrography in a U.S. Geological Survey Bulletin (no. 237) in 1905.

Pirsson’s most-cited contribution is the formulation of an entirely new quantitative classification and nomenclature of igneous rocks on the basis of their chemical and potential mineral composition. Developed in cooperation with C. Whitman Cross, Joseph P. Iddings, and Henry S. Washington, the CIPW system, identified alphabetically by the first initial of the four originators’ last names, provides a method of calculation that results in a set of theoretical, ideal, end-member minerals, that is, “normative” minerals that closely represent the actual minerals crystallized, or if the rock is glassy, minerals that would have crystallized. The system is particularly useful in relating rock types. It has also served as the basis for investigating experimentally synthetic rock systems composed of the principal rock-forming, ideal, end-member minerals, that is, the normative minerals. Although Iddings is given the most credit for devising the CIPW system, it is evident that each of the originators played a critical role based on their broad cumulative knowledge of rocks in the field. Pirsson served primarily as the “moderator” in adjudicating the divergent opinions of the group. First presented in 1902 (Journal of Geology 10: 555–690), the CIPW system is indeed a remarkably perceptive and imaginative scheme that many years of experimental studies on progressively more complex mineral systems have demonstrated to be accurate and descriptive of the relationship of minerals in rocks and the interrelationships of rocks. It continues to be a major influence in petrology both in teaching and experimental studies.

The third notable contribution of Pirsson is his Textbook of Geology, Part I. Physical Geology, first issued in 1915 with Charles Schuchert, who prepared part II, Historical Geology. It was a well-balanced approach to the principal topics of the subject. The second edition of part I, completely revised, was completed a few months before Pirsson’s death. With successive editions, revised by several members of the Department of Geology at Yale University, it became the most widely used textbook of elementary geology in the United States.

Pirsson was honored by election to the National Academy of Sciences in 1913, and he served as vice president of the Geological Society of America in 1915. He held membership in the Geological Society of Stockholm, Sweden, the American Academy of Arts and Sciences, the Connecticut Academy of Arts and Sciences, and the Washington Academy of Sciences. The mineral pirssonite was named in his honor by his friend and associate Joseph H. Pratt of the Sheffield Scientific School in 1896 American Journal of Science, 4th ser., 2: 123–35.

Pirsson has been described as a kindly person with a charming personality. He was thoughtful and careful in reaching judgments and had a keen sense of responsibility. It was presumably for these attributes that he became secretary of the governing board of the Sheffield Scientific School and later chairman of the discipline committee. Although a forceful teacher and fair disciplinarian, his students thought of him as a sympathetic friend and advisor. A colleague portrayed him as the “embodiment of unselfish devotion to duty” and a “true university professor.”

In 1902 Pirsson had married Eliza Trumball Brush, daughter of Professor George J. Brush, former director of the Sheffield Scientific School, his mentor and close friend; the couple did not have children. Pirsson died in New Haven, Connecticut, after a period of intense suffering with arthritis.

Louis Valentine Pirsson was one of the leading petrologists of his generation, a successful teacher and author of a widely used textbook. Pirsson is in the list of the 100 leading men of science in the United States in geology compiled in order of distinction by a peer group in 1903 (published in 1933 in American Men of Science, p. 1273).


Pirsson’s correspondence with C. Whitman Cross, H. S. Washington, and other prominent petrologists of the day, as well as his autobiography dated 1906, are held in the Manuscripts and Archives Division of the Yale University Library. Important works not cited in the text include “Complementary Rocks and Radial Dikes,” American Journal of Science, 3d ser., 50 (1895): 116–21; with Walter H. Weed, “Igneous Rocks of Yogo Peak, Montana,” American Journal of Science, 3d ser., 50 (1895): 467–79; with W. H. Weed, “Missourite, a New Leucite Rock from the Highwood Mountains of Montana,” American Journal of Science, 4th ser., 2 (1896): 315–23; with W. H. Weed, “Geology and Mineral Resources of the Judith Mountains of Montana,” U.S. Geological Survey Report 18, pt. 3 (1898): 437–616; with Henry H. Robinson, “On the Determination of Minerals in Thin Rock Sections by Their Maximum Birefringence,” American Journal of Science, 4th ser., 10 (1900): 260–65; and his five-part “Contributions to the Geology of New Hampshire,” with Henry S. Washington, pt. I, American Journal of Science, 4th ser., 20 (1905): 344–52, pt. II, 22 (1906): 439–57, pt. III, 23 (1907): 257–76, 433–47; and with William N. Rice, pt. IV, 31 (1910): 269–91, and pt. V, 31 (1911): 405–31. Another widely used book by Pirsson is Rocks and Rock Minerals: A Manual of the Elements of Petrology without the Use of the Microscope (1908). A complete bibliography is given by Adolph Knopf in National Academy of Sciences, Biographical Memoirs 34 (1960): 228–48. Knopf had access to an autobiography covering Pirsson’s early life that was made available by his widow. Additional information is provided by C. Whitman Cross’s earlier biographical sketch in the American Journal of Science, 4th ser., 297 (1920): 173–87.