Page:The Journal of geology (IA journalofgeology11893univ).pdf/192

It is evident that descriptive petrography needs some generally accepted term for both a macroscopic and microscopic rock structure which is, in a certain sense, intermediate between those known as the granular or microgranitic and graphic or micropegmatitic. Areas have been observed and variously described in many types of massive rocks, whose component minerals possess neither the complete independence of optical orientation characteristic of granular structures, nor the entire optical continuity of the separated portions of two interpenetrating crystal individuals. 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 reference 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,^[1] although no particular name was at that time given to it. In 1886 the writer proposed the term poikilitic