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.
With a recommendation from George Jarvis Brush, the head of the Sheffield Scientific School, Pirsson joined the United States Geological Survey (USGS) as an assistant in an expedition to the Yellowstone National Park area led by Arnold Hughes.
[1][2] After another geological field season in Montana, Pirsson, supported by an inheritance, traveled in 1891 to Heidelberg and studied mineralogy for one semester with Karl Heinrich Rosenbusch.
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.
With its 1902 publication, the new scheme, named CIPW for the researchers' initials in alphabetical order, was immediately widely adopted.