2(36 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



The changes of crystallization may be traced horizontally in the imme- 

 diate vicinity of the core, increasing from the outlying bodies toward the 

 core, the change being rapid near the core and accompanied by induration 

 and metamorphism of the surrounding rocks. It is in great measure inde- 

 pendent of the size of the body, since narrow dikes within the core are 

 coarsely crystalline, while much broader ones in the surrounding country 

 are very fine grained. 



It was unquestionably the differences in the temperature of the core 

 rocks and of the outlying breccias which determined the degree of crystal- 

 lization. The core was undoubtedly much more highly heated than the 

 surrounding rocks, and the bodies of magma that solidified within it cooled 

 much more slowl} T than those injected into the outlying parts of the vol- 

 cano, or even within a mile of the central conduit. 



From this it follows that the application of the terms "deep-seated" 

 and "abyssal" to coarsely crystalline rocks is misleading, since it is not 

 distinctive and applies equally well to rocks of totally different crystalline 

 characters. The depth at which a magma has solidified appears to be of 

 little moment in comparison with the temperature of the rocks surrounding it. 



DEVELOPMENT OF PHENOCRYSTS. 



A consideration of the various mineralogical phases of rocks which 

 have the same chemical composition, as they occur in this district, leads to 

 important conclusions regarding the crystallization of phenocrysts. The 

 great majority of the basaltic dikes carry porphyritical crystals of olivine, 

 augite, and plagioclase in a microcrystalline groundmass of plagioclase, 

 augite, and magnetite. In other varieties the phenocrysts are almost 

 wholly olivine and augite. Within the core there are rocks with pheno- 

 crysts of augite and plagioclase, sometimes with olivine, sometimes with 

 paramorphs after olivine. The outlines of these crystals show that their 

 crystallization continued uninterruptedly into the . period of crystallization 

 of the groundmass. These rocks are more generally the fine-grained 

 forms. The greater part of the gabbro does not carry olivine, or at least 

 only in occasional crystals, while the augites possess characters different 

 from those in the basalt; besides which, hypersthene and biotite have 

 developed in crystals as large as those of augite. It is also observed that 

 the apatite and magnetite are differently developed, being in larger and 

 fewer individuals in the coarse-»rained rocks. 



