6o6 /. E. SPURR 



The writer, of course, interprets the term ophitic in its broader 

 and not in its narrower sense, accepting it as meaning a structure 

 where a network of interlocking, divergent, comparatively large 

 feldspar crystals is filled in by grains of much smaller dimensions, 

 whatever the nature of these grains may be. He does not inter- 

 pret it as meaning that the mesostasis is necessarily augite. He 

 finds grounds for this broader acceptation in the writings of 

 Rosenbusch, Zirkel, and others. Rosenbusch applies the term 

 ophitic^ to diabases where the mesostasis is not augite, but an 

 aggregate of primary quartz and feldspar. 



Therefore, the glassy, fine porphyritic, coarse porphyritic, 

 ophitic, aplitic and hypidiomorphic granular structures may occur 

 in any rocks. They pass by gradual transitions into one another 

 and are dependent upon relatively very slight differences in con- 

 ditions of cooling. All may be formed without any marked 

 migration of the consolidating rock. 



These conclusions are important in considering rock classifi- 

 cation, as showing that structure cannot be made the element 

 of greatest importance. Granites, granite porphyries, granite 

 aplites and rhyolites, for example, must not be separated, but 

 put as closely together as possible, and the same is true of 

 diabases, diabase porphyrites, diabase aplites, and basalts. 



J. E. Spurr. 



^ Mikroskopiscke Physiographic, 3d ed., p. 11 17. 



