178 THE JOURNAL OF GEOLOGY. 



have exactly the same optical orientation, and must hence extinguish the light 

 between crossed nicols together. Such a structure is termed, according to 

 the particular form it assumes, inicropegjtiatitic or granophyric. 



" In the third place a single large crystal of one of the two constituents 

 of the groundmass may be filled with much smaller, irregularly arranged 

 grains or crystals of the other. This would also give the general effect of a 

 finely granular structure, although it is essentially different from either of the 

 others above mentioned." ' 



The same structure was briefly described by Teall in a quartz- 

 felsite from the Cheviot Hills, but without any particular designa- 

 tion being applied to it.^ Harker also mentions a variety of the 

 same structure as common in the ancient rhyolitic lavas of 

 Wales. 3 Cross described the macro-poikilitic structure in a 

 hornblende-peridotite, from Custer county, Colorado, "^ and the 

 micropoikilitic structure in a rhyolite from Silver Cliff in the 

 same district, although the connection between the two was not 

 mentioned. In speaking of the latter rock, he says of the 

 groundmass : 



"There seems to be no isotropic matter, but individual characteristics of 

 form and optical action are lost through the minute size of the grains which 

 overlap and overlie each other in the thinnest attainable sections. This 

 mixture is irregular in many cases, but in others a mottled appearance is 

 produced in that one substance attains a uniform optical orientation in certain 

 areas, but is filled by inclusions of the other substance. No regular inter- 

 growth of the two can be discovered. In some spots it was clearly quartz 

 which was the enveloping mineral." s 



Brogger has described the groundmass of a quartz - porphyry 

 from the region of Christiana as having a typical poikilitic 

 structure.^ 



In his recent monograph on the Eruptive Rocks of Electric 

 Peak and Sepulchre Mountain in the Yellowstone Park, Iddings 

 describes the micropoikilitic structure in the groundmass of 

 certain dike porphyrites, where he for the first time makes use of 

 exactly this term.^ In speaking of the Sepulchre Mountain dikes, 



' Loc. cit. pp. 367, 368. ^British Petrography, p. 343. London, 1888. 



3 The Bala Volcanic Series. Cambridge, 1889, pp. 22, 23. 



"• Proc. Colorado Scientific Society, vol. 2, p. 242. 1888. 



5 lb., p. 232. * Zeitsch. fiir Krvst. u. Min., vol. 16., 1890., p. 46. 



7 Twelfth Ann. Rept. U. S. Geol. Survey, p. 589. 1892. 



