Page 74 - Petrology of Sedimentary Rocks

extinction, inclusions, and grain shape. This is always highly subjective, often difficult
and requires considerable experience. However it is the ultimate aim of the study of
quartz types and the most valuable system of classification because through it the
character of the source area can be determined, and much valuable paleogeographic
interpretation can be made. In the empirical classification, a binomial series of
pigeonholes is set up based on 6 different types of extinction and 4 types of inclusions,
giving a total of 24 arbitrary classes. These classes tie up approximately with the
genetic classification, but there is considerable overlapping (for example, straight
extinction quartz with few bubble inclusions may come from plutonic, recrystallized
metaquartzi te, or vein environments). This is an objective classification, is easily
learned and hence, is amendable to statistical techniques and the comparison of sets of
samples by means of the X2 test. Statistical tests are difficult to apply to the genetic
classification because of its subjectivity. The best procedure is to count a hundred or
so grains using the empirical classification, then based on this, and with the genetic
classification in mind, conclude about the provenance of the grains.

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