VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 605 



effusive bodies, but further down, at the roots of volcanoes, the 

 conditions are such as to allow viscosity to increase with the 

 same slowness that it does with a more rapid cooling rate in 

 more basic rocks. Hence with increasing acidity we find the 

 coarser grained varieties further removed from the surface. In 

 general, however, it is plain that a granular rock is not necessarily 

 a deep-seated one, in the formerly accepted sense of the word. 

 Another conclusion which may be made from the foregoing 

 studies is, that the more important structures are not peculiar 

 to particular rocks. The porphyritic and the coarse granular 

 allotriomorphic or hypidiomorphic structures are already recog- 

 nized as characterizing all rocks, of whatever chemical composi- 

 tion. Also the aplitic structure, or that in which idiomorphic 

 minerals (which are the same as the phenocr3''sts of the por- 

 phyries) form the greater bulk of the rock, has been recognized 

 as universal by Rosenbusch, who has described it in granitic rocks 

 and in all intermediate ones down to gabbros ; thus his rock 

 terms include syenite aplite and gabbro aplite. The ophitic 

 structure has been generally supposed to be characteristic of 

 diabases, and without question is here best exhibited, on account 

 of the rate of solidification which the basic composition of a 

 magma entails and also because the elongated forms of the basic 

 feldspars make the structure prominent. The foregoing studies, 

 however, show that this structure is intermediate between the 

 porphyritic and the aplitic structures, representing a stage in 

 crystallization when the idiomorphic crystals (or phenocrysts, 

 as they are called in the porphyries) have multiplied and grown 

 so that they interlock ; and that like these other structures it 

 may occur in any rock. In the granites it is not so striking as 

 in more basic rocks, on account of the blunt form of the alkaline 

 feldspars which form the first generation of crystals, but it is 

 nevertheless present in some of the granites which have been 

 studied. In diorites the ophitic structure has been occasionally 

 described.^ 



'RosENBUSCHC, Op. cit., p. 256; ZiRKEL, Lehrbiich der Petrographie, 2d ed., 

 p. 483. 



