68 



University of California. 



[Vol. i. 



I. Porphyritic fades. Berkeley; analyst, C. Palache. 



II. Spherulitic facies, porphyritic variety, Berkeley; analyst, 

 C. Palache. 



III. Glassy facies, porphyritic variety, Berkeley; analyst, C. Pa- 

 lache. 



IV. Lithoidal rhyolite,* Yellowstone National Park. 



V. Quartz-keratophyre.t Mt. Elizabeth, Australia, Howitt. 



An inspection of the three analyses of the Berkeley rock shows 

 that its different facies are not identical in composition. The silica 

 content varies in a pronounced way, and presents a gradation 

 decreasing with the grade of crystallization. The specific gravity, 

 also, decreases in value with the decrease in silica, a fact contrary 

 to the general rule. 



The explanation of this anomalous variation of the density of 

 the rock is doubtless to be found in the increase in the proportion 

 of glass (fused quartz, resolidifying as glass, being lighter than 

 crystalline quartz J), and in the fact that the glassy rock is rich in 

 water. 



The porphyritic rock contains more lime than the other two 

 facies, and also differs from them in the small proportion of alumina 

 which it contains, the amount being less than would be expected 

 from the amount of alkalies and lime, with which it is presumably 

 combined in the form of a silicate. 



The variation in the silica content is interesting and suggestive, 

 and seems to be an illustration of the operation of Soret's principle, 

 now generally accepted in discussions § on pedogenesis. 



Those portions of the rock which are holocrystalline, and which, 

 therefore, have presumably solidified most slowly, are richest in 

 silica, while the glassy facies which solidified rapidly is most basic. 



*J. P. Iddings, Origin of Igneous Rocks, Bull. Phil. Soc. of Wash., Vol. 

 XII, June, 1892, p. 203, Table I, analysis 26. 



t H. Rosenbusch, Ueber die chemischen Beziihungen der Eruptivgesteine. 

 Mineralog. und Petrog. Mitth Vol. XI, 1S89, p. 177, Table I, analysis XX. 



jTeall, J J. H , British Petrography, iSSS, p. 12. 



JTeall, ]. J. H., The Sequence of Plutonic Recks, Natural Science, Vol. I, 

 June, 1892, pp. 288-300. 



Iddings, J. P., The Origin of Igneous Rocks, Bull. Phil. Soc. Wash., Vol. 

 XII, pp. 89-214, June, 1892. 



