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Reduction spots are small greenish to yellowish spots in slate produced by reduction of iron. If initially spherical these passive markers may be useful as passive strain markers. Reduction spot studies suggest that total shortening strain (e1) associated with slaty cleavage is >60%. This work was carried out by Dennis Wood, the principal locality being the slate belt in Anglesey. Since in case of deformed reduction spots Rxz was found not to be equal to the product of Rxy and Ryz, the thesis of volume loss during the formation of most of the slate belts was born. Since Flinn diagram was based on volume conserving deformation, logarithmic Flinn plot was introduced with the principal oblate and prolate fields were indeed those of apparently oblate and apparently prolate.