Louis V. Pirsson

Louis Valentine Pirsson (November 3, 1860 – December 8, 1919) was an American geologist and petrologist, best known for devising (with collaborators Charles Whitman Cross, Joseph Paxson Iddings, and Henry S. Washington) the CIPW norm for classifying igneous rocks. (He is the P in CIPW.) His 1915 Textbook of Geology was for decades the most widely used in the United States.

Biography Pirsson was born in Fordham, New York, to parents Francis Morris Pirsson and Louise M. (nee Butt) Pirsson. His mother died when he was four, and he was left in the care of cousins. When his foster parents left in 1869 for a journey lasting several years, the boy was left with a minister, from whom he received a classical education in mathematics, geography, Latin and ancient Greek. During this time he developed an interest in the natural sciences, particularly ornithology. From the age of 16 he attended a boarding school in Amenia, New York, whose principal was a Yale graduate. He was encouraged his student to continue his studies there.

CIPW Norm

Pirsson's intensive study of rock microscopy and petrography repeatedly raised problems in the classification of magmatic rocks, which at the time was often inconsistent. In 1899, the four petrologists Cross, Iddings, Piersson, and Washington met to develop a new classification based on quantitative chemical analysis. The idea was to develop a rigorously quantitative scheme that was independent of a rock's crystallization history (as it cooled) but that would reflect the characteristics of the assumed homogeneous melt from which it derived. Pirsson mediated among the different positions of his colleagues and brought the project to a successful conclusion. With its 1902 publication, the new scheme, named CIPW for the researchers' initials in alphabetical order, was immediately widely adopted.