POIKILITIC AND MICROPOIKILITIC. I J J 



[ttolklXos, mottled)' 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 scJiillerfeh of Germany. This had before been 

 called '' histcr mottli?ig," by Pumpelly^ and Irving, 3 but this name 

 is not capable of application to other allied structures of different 

 appearance. In 1887 the writer described thfs macro-poikilitic 

 structure in the orthoclase phenocr^'sts of an orthoclase-norite, 

 belonging to the Cortlandt series. ^ 



Though it is not uncommon in many minerals, it is less im- 

 portant and less frequent than the micropoikilitic structure in the 

 groundmass of acid porphyritic rocks of all ages. When study- 

 ing the ancient quartz-porphyries of Missouri for his thesis, Prof. 

 E. Haworth encountered it and applied to it for the first time 

 the nz.n-\.e poecilitic.^ 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 holocrystalhne structure 

 are distinguishable. These three types are conditioried by the mutual rela- 

 tion 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 



'American Journal of Science (3"* ser.), vol. 31, p. 30, Jan., 1886. This term was at 

 first incorrectly spelled poiciiitic and subsequently corrected by Prof. Dana to its Latin 

 iorm, poeciliHc {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 Breit- 

 haupt'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, n., p. 38). The totally 

 different application of these ternns could, however, produce no confusion with the 

 one now proposed, even if they were not obsolete. 



= Proc. Am. Acad., vol. 13, p. 260. Boston, 1878. 



3Monogr. U. S. Geol. Survey, vol. 5, p. 42, 1883. 



''American Journal of Science, (3'' ser.) vol. 33, p. 139, 1887. 



5_Am. Geologist, vol. i, pp. 368, 369; PL I, fig. i, June, 1888. 



