SLICING ROCKS THIN

Evening Post, Volume CXXV, Issue 130, 4 June 1938, Page 27

SLICING ROCKS THIN

No matter how thin you slice it, it's still—rock, says the "San. Francisco Chronicle."

Geological f survey specialists have developed a technique for slicing the toughest, hardest boulder thinner than the proverbial restaurant beef-steak. Actually, the thinnest measures but one-thousandth of an inch.

Only three or four laboratories in the United States are able to duplicate the process used in Washington, and the only foreign competition is from three laboratories located in England, Germany,, and Norway. Thinner than tissue paper and far more transparent, the specimens are mounted on microscope slides.

Between three and four thousand specimens are cut, ground, and polished every year, the survey in most cases being asked to determine the mineral content of the rock from Which the specimen came. This is a far cry from the old-time prospector's hit-and-miss selection of mineral lands.

The making of microscopic slides of rock sections is an occupation as delicate as the jeweller's and as obscure as the curling of willow plumes. Slicers Frank Reed and John Mergner learned their trade as apprentices of a naturalised Dane who was employed by the Geological Survey twenty years ago. He learned how In Norway.

No slicing job is too difficult for

them. Take a husky boulder from the rocky gorge of Grand Canyon, for instance. The geologist tramping the gorge knocks off a sample with his prospectors' pick and sends it to Heed and Mergner in Washington.

With their rock saws they slice a piece from the sample, preferably a very small piece. This is ground with carborundum powder of varying , degrees of coarseness and fineness, until a smooth surface is attained. The surface is then cemented with Canada balsam to a glass slide. Next, the other side of the specimen is ground until it reaches the required thinness of onethousandth of an inch. Finally, a protecting glass, cover is cemented with balsam to the exposed surface and the slide is ready f?r study.

Minerals and rocks may be just jagged, unattractive masses when viewed in their native form, but under the microscope, ground and polished, they take on the gay, transparent qualities of gems from fairyland. Under the powerful lenses of the microscope they show patterns and designs more intricate than snowflakes, with colours brilliant and varied.

In addition to preparing thin sections of rock, these skilled technicians also prepare mounts of' polished ores for study by reflected light. Such things as gold flecks smaller than one one-hundredth-thousand of an inch can be detected and examined.