POIKILITIC AND MICROPOIKILITIC. 1 79 



he calls it a "patchy structure," but says it is identical with what 

 he before called the micropoikilitic (micropoicilitic).^ 



The micropoikilitic structure is extremely abundant in the 

 ancient acid lavas of South Mountain,^ in southern Pennsylvania 

 and Mar_yland. It can there be proved in some cases to be ot 

 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 solidifi- 

 cation of the rock. 



I am not aware that either the macro- or micropoikilitic struct- 

 ures 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 

 Fouque examined and pronounced an admirable example of the 

 '' type epongeuse" sometimes called ^^ structure petrosiliceuse a ponce T 

 It is clearly not the same as Michel Levy's structure globulaire, 

 which he defines: "Spherolites radies, impregnes de quartz 

 oriente dans un sens optique unique. Globules impregnes de 

 quartz oriente, aureoles, "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 sufificient 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. 



George H. Williams. 



'lb., p. 646. 



= American Journal of Science, (3'' ser.) vol. 44, p. 482, Dec, 1892. 



3 Roches Eruptives, p. 29. Paris, 1889. 



