How did we first accurately identify rocks?

By slicing rock into extremely thin sections and studying them with a polarising microscope, Henry Sorby revolutionised the science of geology.

24th July 2017
How did we first accurately identify rocks? © Getty Images

Henry Sorby pioneered the study of geology and rock identification by using a polarising microscope to examine rocks, an idea he got from a chance meeting on a train with a Manchester surgeon who taught him to make sections of fossil wood, teeth and bones. It occurred to Sorby that a great deal might be learned by applying a similar method to the study of rocks. He made numerous thin sections by first grinding a slice of rock roughly, smoothing it on a lead plate with coarse emery, and finishing on a copper plate with fine emery. The thickness of 30 microns (millionths of a metre) that he achieved is still the standard used today, over 160 years later.

In 1851, Sorby published a paper describing this technique and the mineral content of a thin section of sandstone. Using a microscope fitted with polarised light, he determined that some of the particles were calcite, some were quartz and others agate, which would have been impossible using the old method of crushing rocks. Unfortunately, this new technique of examining rocks did not find favour among most geologists of the day and was ridiculed by many. It wasn’t until the 1860s that the technique of using polarised light to identify minerals became established as an essential tool for the examination of rocks.

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