Subject: Feldspar twins. (1) An appeal for their study from the point of view of their possible exploitation. Can they be used as a basis of igneous rock correlation? What decides which twin laws shall prevail? (2) A description is given of a new, very rapid procedure for the determination of the twin laws. We estimate the time needed for the determination of such twin laws by the new“five-axis method” to be definitely less than 10% of the time required for the original four-axis technique of Fedorov. There is also afforded an increase in both accuracy and range of applicability.

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