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Andrew Alden

What Are Plutons?

By September 16, 2013

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I spent the weekend coming up with a good answer to that question, which is now in my glossary. But I couldn't include the story about plutons told by Charles B. Hunt. In the 1930s, Hunt revisited the classic Henry Mountains of eastern Utah where G. K. Gilbert, 50 years earlier, described the plutons known as laccoliths ("our survey has been described as having determined in five years what Gilbert discovered in two months"). His subsequent report, USGS Professional Paper 228, defined a wacky kind of pluton called a cactolith. Here's what he said about it in 1987 (in GSA Memoir 167):

It seems appropriate to record here the origin of the term cactolith . . . It was intended to call attention satirically to the absurd nomenclature geologists were developing by applying new names to the infinite variety of shapes intrusions can form. The name cactolith and its definition started July 1939 at what may be called elegantly a luncheon seminar on the outcrop of the feeder to the Trachyte Mesa laccolith. . . . Attending that seminar were Drs. J. W. Gregg, Frank Schairer, Earl Ingerson, and E. F. Osborn of the Carnegie Geophysical Laboratory and Dr. N. L. Bowen, then at the University of Chicago. I took the notes. Dr. Gregg proposed the name because the intrusion's shape so resembled the woody structure of the cane cactus. The definition that evolved from the discussion described it as a "quasihorizontal chonolith composed of anastomosing ductoliths whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith." I personally think that Gilbert would have approved even though the New Yorker did refer the definition to their "How's that again" department.

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