PETROGRAPUY. 331 



These results, though extremely suggestive, are not, it will be ob- 

 served, in exact accordance with those arrived at by Messrs. Diller and 

 Iddings, to whose papers we have already referred. The results ob- 

 tained by these gentlemen, coupled with what is already known regard- 

 ing the phenomena of crystallization, seem to show that structural feat- 

 ures, and to a certain extent the universal composition of eruptive rocks, 

 are dependent upon (1) the chemical composition of the magma and (2) 

 the varying conditions of heat and i)ressure under which this magma 

 existed prior to and during the course of its eruption. A mineral which 

 separates out under certain temporary conditions of heat and pressure 

 may, under changed conditions, be partially or wholly resorbed, and the 

 condition of chemical equilibrium be so changed as to give rise to crys- 

 talline secretions of quite different nature. 



Mr. G. F. Becker, in a paper treating of the texture of massive rocks,* 

 and drawing his illustrations from the maguiticent exposure offered by 

 Mount Davidson and the Comstock Lode, IsTeviida, argues on chemical 

 and theoretical grounds that the structural differences existing between 

 holocrystalline porphyritic and granular rocks are due not to condi- 

 tion of cooling, but rather to original differences in composition and 

 fluidityof the magma. Granular structure, excepting in rarecases,isdue 

 to imperfect fusion and fluidity of the magma. Pori)hyritic structure, 

 on the other hand, is the normal structure of rocks cooling gradually 

 from a high temperature and consequent state of very ])erfect fluidity. 

 When the two types of structure are associated it is argued that this is 

 due to a lack of homogeneity in chemical composition and to tempera- 

 tures sufScient to fuse portions, but not the entire mass. Granular 

 rocks as a rule, he argues, have formed at lower temperatures than por- 

 phyriesof precisely the samechemical composition. Granularstructure is 

 therefore characteristic of rocks formed by the metamori)hism of sedi- 

 ments and porphyritic structure characteristic of those crystallizing 

 from homogeneous fluid magmas. These conclusions are based largely 

 on a new law of thermochemistry advanced by Mr. Becker in a previ- 

 ous paper.t In this same connection may be mentioned the researches 

 of Professor Judd, who, in a very interesting and important paper in 

 the Geological Magazine for January, 1888,:j: calls attention, first, to the 

 great dissimihirity in chemical coniposition of rocks classed as enstatito 

 or hypersthene ardesite, the silica percentages in extreme cases varying 

 nearly 20 per cent., and shows that this diff"erence is due mainly to the 

 relative pi'oportions of tiie crystalline constituents to the glassy base. 

 Then, treating wholly of the glassy lavas of Krakatoa, he shows that 

 the obsidian, containing a considerable percentage of volatile matter, 

 swells up and fuses at approximately a white heat, while the stony 



* Am. .Jour, of Sci., 1887, xxxiil, p. .')0. 

 • t Am. Jour, of .Sci., 1S8(), xxxi, p. 1">0. 



t Thti Natural History of Lavas as illusfratiHl by the materials ejected from Kra- 

 katoa. J. W. Judd. Geol. Mag., January, J888, vol. V, p. 1. 



