Page:The Journal of geology (IA journalofgeology11893univ).pdf/195

he calls it a "patchy structure," but says it is identical with what he before called the micropoikilitic (micropoicilitic).^[1]

The micropoikilitic structure is extremely abundant in the ancient acid lavas of South Mountain,^[2] in southern Pennsylvania and Maryland. It can there be proved in some cases to be of secondary origin as it occurs in plainly devitrified glasses, and it is the writer's opinion that such enclosing quartz areas will, in many cases, prove to have originated subsequent to the solidification of the rock.

I am not aware that either the macro- or micropoikilitic structures have been directly recognized by the German petrographers. I am indebted to Prof. L. V. Pirsson of New Haven for the information that the latter is recognized in France, although we have been unable to find any definition of it in print. He has shown me a section of a quartz-porphyry from Georgia, with a groundmass exactly like those from South Mountain, which Fouqué examined and pronounced an admirable example of the "type épongeuse," sometimes called "structure pétrosiliceuse à ponce." It is clearly not the same as Michel Lévy's structure globulaire, which he defines: "Sphérolites radiés, imprégnés de quartz orienté dans un sens optique unique. Globules imprégnés de quartz orienté, auréoles,"^[3] because there the included matter is radially arranged, while in the micropoikilitic structure it is wholly irregular in its arrangement.

The references given above are sufficient to demonstrate the frequency of the rock structure here mentioned, and to show the desirability of some term to describe it. It is therefore proposed that poikilitic and micropoikilitic be employed for rock structures, whether primary or secondary, conditioned by comparatively large individuals of one mineral enveloping smaller individuals of other minerals, which have no regular arrangement in respect either to one another or to their host.