THE CONTROVERSIAL GRANITE CONTROVERSY

The granite controversy could be said to be fought to a draw. As many other rocks have given up their secrets via experiment and field work over the years, the result of searching for an igneous or metamorphic origin for granite has resulted in a spectrum of answers. The earliest origin conundrum courtesy of J. Hutton and A.G. Werner in the 18th century, was via fusion from heat or crystallization from solution. Quartz, the most difficult of granite minerals, could not be fused, but could be, and was, put into solution and crystallized out. Hutton's field evidence at Glen Tilt did not convince everyone. Differences of melting temperatures of quartz and feldspar indicated that quartz should crystallize first, but instead feldspar is defined, with quartz fitted around it. The laboratory could not produce granite melt or texture, but because of the laboratory work of R. Kirwan and others, Werner's theory of solution origin for granite and basalt retained meaning well into the 19th century. Gneisses and schists, with approximately granite composition, were a problem: the category of metamorphic was added, especially by C. Lyell. Whether foliation was a product of altered sedimentary layers or deformation by pressure (or both) was a question. Field work in Canada showed foliated metamorphic rocks interleaved with sedimentary rocks. The increasing use of thin sections posed many conundrums. As well, the existence of a granitic magma was questioned despite great granite plutons on several continents. A partial metamorphic origin was suggested, and the stage was set for the controversies of the late 19th century and through the 20th. Granite could not be synthesized from its constituents either by fusion or in the presence of water. Field work showed it often graded into gneisses. Sometimes granite intrusions didn't deform surrounding rocks, leading to the "space problem." While there were many geologists intrigued with the problems, some prominent players were N.L. Bowen with his crystallization differentiation, the well-recorded evolution of H.H. Read's ideas, H.S. Yoder's experimental work, and W.S. Pitcher's thoughtful evaluation of field sites. Words that came into use included migma and migmatite.