The distinction between quartz and untwinned oligoclase-andesine in thin section, or powder, is difficult to make, and is a problem not infrequently encountered in petrographical work. The concurrence of the indices of these two minerals makes their separation by means of index liquids impractical. The use of interference colors is untrustworthy. Interference figures, while capable of affording a qualitative answer, can in no way be relied upon to furnish even an approximate quantitative answer without hours of work, and where the number of grains is limited (as in a thin section), or where the grain size is very small, they are useless.

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