2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 158-10
Presentation Time: 4:00 PM


AALTO, K.R., Geology Dept, Humboldt State University, 1 Harpst St, Arcata, CA 95521, kra1@humboldt.edu

Ferdinand Zirkel (1838–1912), a German world leader in the mid-19th Century petrographic study of rocks, was engaged by Clarence King (1842–1901), Geologist-in-Charge of the Geological Exploration of the Fortieth Parallel, to study survey specimens. Zirkel prepared some 2,500 thin sections and wrote Microscopical Petrography (v. 6 of survey reports, 1876), which constituted a first introduction of this science into American geological studies. Included are twelve plates, with superb hand-drawn and -colored images, chiefly of volcanic rocks in thin section. Visual components include mineral crystals, microlites, fluid inclusions, crystal rim fabrics and groundmass textures with accompanying discussion of their origins. Zirkel (1876, p. 5) concluded “the large and small crystals were not formed exactly where we perceive them, but that they have been thrown into their present place by the purely mechanical action of the surrounding plastic mass.” Following King, Zirkel accepted the natural system of volcanic rock classification proposed by Ferdinand von Richthofen (1833–1905), who in 1862 traveled to California to study volcanism. He thus accepted von Richthofen's designation of pre-Tertiary ‘propylite’ as a volcanic rock species universal precursor lithology. Propylite was later determined to be simply hydrothermally altered andesite and discarded as a unique species of volcanic rock. Nonetheless, the visual impact of Zirkel's sketches launched a new era of microscopic study of rocks in North America.