There is a recent tendency in the geological sciences to introduce the quantitative factor in laboratory investigation. In petrography the development of the micrometer stage and more recently the development of electric counters for thin-section analysis has greatly facilitated the modal determination of rocks. Larsen and Miller have studied the accuracy and limitations of Rosiwal analyses and outlined the conditions as to grain size, area of thin section and number of traverses, under which the mode of a rock can be determined by the Rosiwal method with an accuracy of 1 per cent.

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