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POIKILITIC AND MICROPOIKILITIC.
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(ποικὶλος, mottled)[1] for the macroscopic equivalent of this structure which is characteristic of the hornblende of the Stony Point hornblende-picrite or cortlandtite, as it is also of the Baste and Schriesheim schillerfels of Germany. This had before been called "luster mottling," by Pumpelly[2] and Irving,[3] but this name is not capable of application to other allied structures of different appearance. In 1887 the writer described this macro-poikilitic structure in the orthoclase phenocrysts of an orthoclase-norite, belonging to the Cortlandt series.[4]

Though it is not uncommon in many minerals, it is less important and less frequent than the micropoikilitic structure in the groundmass of acid porphyritic rocks of all ages. When studying the ancient quartz-porphyries of Missouri for his thesis, Prof. E. Haworth encountered it and applied to it for the first time the name poecilitic.[5] In this connection the writer furnished Dr. Haworth the following from his lecture notes:

"A holocrystalline groundmass contains no amorphous or unindividualized matter whatever, and independently of differences occasioned by variations in the fineness of grain, three quite distinct types of holocrystalline structure are distinguishable. These three types are conditioned by the mutual relation of the quartz and feldspar crystals, which compose the groundmass. In the first place they may be wholly independent, thus giving rise to a granular aggregate which is well designated by the term Microgranitic Structure.

"In the second place a granular effect may be produced by the complete interpenetration of two individual crystals of the same size. In this case—due to the simultaneous crystallization of the two minerals from the magma all the parts of the same individual, no matter what the size or shape, must
  1. ↑ American Journal of Science (3d ser.), vol. 31, p. 30, Jan., 1886.This term was at first incorrectly spelled poicilitic and subsequently corrected by Prof. Dana to its Latin form, poecilitic (ib. vol. 33, p. 139, Feb., 1887). Its preferable orthography is, however, that given above. At the time it was proposed the writer was not familiar with Breithaupt's name, poikilit, for bornite, nor with the designations, terrain poecilien, poecilitic and poikilitic, given successively by Brongniart (1829), Conybeare (1832) and Buckland (1837) to the "New Red" sandstone (cf. Bridg. Treat. 11., p. 38). The totally different application of these terms could, however, produce no confusion with the one now proposed, even if they were not obsolete.
  2. ↑ Proc. Am. Acad., vol. 13, p. 260. Boston, 1878.
  3. ↑ Monogr. U. S. Geol. Survey, vol. 5, p. 42, 1883.
  4. ↑ American Journal of Science, (3d ser.) vol. 33, p. 139, 1887.
  5. ↑ Am. Geologist, vol. 1, pp. 368, 369; Pl. I, fig. 1, June, 1888.
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