ON THE USE OF THE TERMS POIKILITIC AND 

 MICROPOIKILITIC IN PETROGRAPHY. 



It is evident that descriptive petrography needs some gener- 

 ally accepted term for both a macroscopic and microscopic rock 

 structure which is, in a certain sense, intermediate between those 

 known as the granular or Tnicrograiiitic and graphic or fnicropeg- 

 matitic. Areas have been observed and variously described in 

 many types of massive rocks, whose component minerals possess 

 neither the complete independence of optical orientation charac- 

 teristic of granular structures, nor the entire optical continuity 

 of the separated portions of two interpenetrating crystal individ- 

 uals. These areas are in fact occupied by a comparatively large 

 individual of one mineral which is more or less completely filled 

 with crystals or grains of other minerals, arranged with no refer- 

 ence to one another or to their host. This structure does not 

 usually appear as distinct from the granular except when seen as 

 a mottling of a large cleavage surface of the enclosing mineral 

 in a hand specimen, or as an irregular spotting of a uniformly 

 extinguishing area under the microscope. In ordinary light, such 

 an area may appear quite granitic, but between crossed nicols it 

 is very distinctive. 



Like the graphic or micropegmatitic structure, this relation 

 is most commonly observed between quartz and feldspar, 

 especially in the groundmass of quartz-porphyries ; but, like 

 that structure, it is also by no means uncommon between many 

 other species. 



Essentially this structure was figured and described at length 

 by the writer in a quartz-porphyry from near Tryberg, in the 

 Black Forest, in 1883,' although no particular name was at that 

 time given to it. In 1886 the writer proposed the term poikilitic 



I Neues Jahrbuch fiir Min., etc., Beilage bd. 11, p. 607. Plate XII, figs. 3 and 3*, 



1883. 



176 



