National Geographic : 1966 Jan
of atoms. As did Alice with her looking glass, I use this instrument to enter another world-a micro scopic realm of dancing colors and shapes. Nature builds rocks with tiny particles of min erals. Most minerals crystallize into characteristic shapes and boast a distinctive set of optical proper ties. Like lenses and prisms, the crystals bend light and break it into separate rays. The color of a mineral seen through the petro graphic microscope depends on the mineral's align ment with the plane of polarization. As I rotate a thin section in polarized light, the mineral align ment continually changes, and I can see a constant ly shifting assortment of colors. It reminds me of a child's kaleidoscope. Gleaming diamonds, rubies, sapphires, emeralds, and opals in thin sections appear almost colorless, to the amazement of my new students in petrog raphy at San Fernando Valley State College. But shimmering beauty may lurk in a drab, lichen covered stone that nicks your lawn-mower blade. By systematically studying samples ground to uniform thickness, geologists have determined the characteristics of every mineral. Now we can quick ly learn a rock's mineral components by observing the colors and shapes of thin-section grains. The relationship of different crystals to one an other and changes in them caused by chemical re actions, heat, or pressure also tell important stories. Sometimes they lead us to the hiding places of pe troleum or metal ores. While geologists employ petrographic analysis to study earth's crust, other professions as well find it helpful. In the police crime laboratory, it can turn up valuable clues, such as the source of dirt parti cles on a suspected slayer's shoes. It helps military researchers assess the sea ice of potential Arctic airstrips. When scientists of Project Mohole recover the first sample of earth's mantle from three miles beneath the ocean floor, they will prepare thin sec tions for petrographic inspection.* Undoubtedly, the first men on the moon will gather rocks for study under the petrographic microscope. For me, the colors and patterns that unfold the mysteries of matter turn a tedious task into grip ping adventure. And when I hear artists declare they have broken the bonds of objective art, I know nature really did it first. *Samuel W. Matthews described the first steps of Project Mohole in the November, 1961, NATIONAL GEOGRAPHIC. Close-up of a stained-glass window? Farmland from 30,000 feet up? Neither. Rose, orange, and mottled pink identify augite. Geologists recog nize the other colors as hornblende, usually green or brown in thin section. Combined, the two min erals produce pyroxene amphibolite-a black rock to the naked eye. KODACHROMEBY LORENCEG. COLLINS © N.G.S. The author films nature's abstracts with a camera atop his petrographic microscope. Dr. Collins uses his slides in classes at San Fernando Valley State College, Northridge, California, where he teaches petrography rock description and classification. Rock thin section appears drab in ordi nary light. Polarized beams kindle spar kling hues in its tiny crystals. Technicians prepare thin sections by slicing wafers from rock samples with diamond saws. They ce ment the wafers to glass slides, then grind each to a thickness of 1/1000 of an inch.