'4 



University of California. 



(Vol. i. 



minerals. That is to say, there appears in each case to have ar- 

 rived a stage in the progress of crystallization of the magma at 

 which the growth of the phenocrysts ceased, while the inclusions, 

 which were being simultaneously formed, continued to separate, and 

 continued out as the ground-mass of the rock, so that on the periph- 

 ery of the phenocrysts it is difficult to say whether the crystals 

 of feldspar, quartz, and mica, which are partly imbedded in the 

 phenocryst and partly protrude beyond its surface, are to be called 

 inclusions, or ground-mass. The continuity of growth of the in- 

 clusions of the phenocrysts into the ground-mass seems to be un- 

 questionable. But with the arrested growth of the phenocrysts two 

 changes took place in the crystallization of the material which 

 yielded both inclusions and ground- mass: (i) It ceased to assume 

 idiomorphic forms, and developed immediately an allotriomorphic 

 granular structure. (2) The size of the constituent minerals of this 

 allotriomorphic granular ground- mass became very much larger 

 than the idiomorphic forms deposited as contemporaneous in- 

 clusions in the phenocrysts. 



It is difficult to say whether the inclusions are, as a rule, more 

 abundant toward the periphery of the phenocrysts than in their 

 central parts, but in some cases this seems to be so, especially as 

 regards the biotite. The phenocrysts are comparatively free from 

 ordinary minute interpositions and liquid inclusions. 



With regard to the ground-mass a word or two remains to be 

 added to what has already been said. There is apparently as much 

 plagioclase as orthoclase in this portion of the rock ; perhaps there 

 is an excess of plagioclase, the latter proportion being indicated, 

 also, by the analyses of the rock given below. The plagioclase is 

 probably oligoclase for the most part. Microlites of apatite may 

 frequently be observed, and minute interpositions and liquid in- 

 clusions are not uncommon in both feldspar and quartz. 



Muscovite is practically absent from the ground-mass, although 

 an occasional plate may be here and there detected, and the rock 

 would ordinarily be classed as a porphyritic granitite. The pre- 

 ponderance of soda and lime in the bulk composition of the rock, 

 together with the corresponding abundance of plagioclase in its 

 mineralogical composition indicate its affinity with the quartz- 



