Thursday 12th October

At the last meeting Tony Thorp gave an illuminating talk on geological thin sections, the history and practicalities. It was Henry Clifton Sorby (1826-1908) whose work laid the foundations of microscopical petrology which became the cornerstone of geology. A man of independent means, he took an interest in microscopy and following in the footsteps of William Nicol and sir William Brewster he learned to make transparent sections of specimens like teeth and bones for observation under a polarising microscope. Of greater importance was the fact that he made thin sections of rock and was the first person to appreciate it's value. His thin sections were 0.03mm (30μ) in thickness which is now the standard. Tony went on to explain the use of the polarising microscope and the property of birefringence. In other words, as polarised light passes through a thin section the different minerals both refract and often re-polarise the light, so that light waves leaving a section are no longer aligned. By inserting a second polariser at 900 to the first all lightwaves that have not been re-polarised will be cut off and appear black. But any that were re-polarised will give a strong colour. Hence the patterning seen when observing a section. Finally advice was give on how to make your own sections at home. I should add that Tony's ingenuity on achieving this was remarkable.

At the next meeting Michele Becker will give the ins and outs of bricks and brickmaking.

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Siderite.

Siderite.

Cafartha mine, Nr Staylittle, Central Wales ore field. This brecciated material is mostly siderite with small pieces of host rock, and a small amount of calcite. The siderite, which is an iron-carbonate is very pale brown in colour, with a slight curvature on exposed surfaces. There are no actual crystals on any of the specimens.
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