PETROGRAPIiy. 329 



well .IS the Britisli student is to be conjiiatulated upon its a])i)earanee. 

 Other new works in book forni, but which rlie i)res(Mit writer not hav- 

 ing seen can speak of only by title, are noted in the biblio<;"rai)]iy. 



ADVANCE WORK, 



While a very large portion of the work of the past two years, as in 

 years before, has been pnrely descrii)tive of the mode of occurrence;, 

 structure, mineral and chemical composition of rocks, yet the snbject of 

 the origin of rocks and the ca^ises of their structural variability have 

 been by no means ignored, as will be noticed. 



THE PHYSICAL AND CHEMICAL CONDITIONS OP CRYSTALLIZATION. 



For many years there has been a growing feeling among geologists 

 that the importance attached, particularly among (Terinan autiiorities, 

 to geological age as a criterion in ro(;k classification was greatly over- 

 estimated, and the fact that neither mineral composition nor structure 

 are uecevssarily dependent upon age is now very generally conceded. 

 In his latest work, to be sure. Professor Kosenbusch has not wholly dis- 

 carded the age (lualiflcation, owing to the fact that it is. as a rule, only 

 among the most ancient rocks that the deep-seated portions have been 

 rendered accessible by erosion, while on the other hand it is often oidy 

 among the more recent that the effusive portions have escaped erosion 

 or alteration and hence are accessible for investigation. The now well- 

 known researches of Messrs. Hague and Iddings ut)on the rocks of the 

 Cornstock Lode, Nevada,* showing that the " degree of crystallizati(m de- • 

 vcloped in igneous ro(;ks is mainly de])einlent uj)on the conditions of lu^at 

 and i)ressure under which the mass has cooled and is independent of 

 geological time," have received abuiulant confirmatory evidence, and it 

 seems now a well-established fa<;t that under similar circumstances crys- 

 tallization and structure may be the same regardh'ss of geological a,ge. 

 More recent discoveries and studies in iliis sanu' general line have been 

 productive of very interesting results. Fn a pajx'r entitlecl " On the lat- 

 est volcaiii(; eruption in Calitbrnia and its ix'cnliar lava,''t Mr. -T. S. Dil- 

 ler has described a very interesting type of ro<-k, evidently a true ba- 

 salt, but uni(]ue in carrying primary porphyritic quartz, associated with 

 olivine, a <;omlition of affairs ordinarily coiisidenMl on (;Iiemical grounds 

 as not likely to occur. The peculiarity is exjjlained on the sut)j>i»sition 

 that the quartz was the first mineral to sepaiate out from the magma, 

 and its crystallization took pla(!e under great pressure at siu'h depth 

 and under such conditions of physi<-al an<l chemical ecpiilibrium as are 

 as yet largely conjectural. Mr. Oilier has been followed by Mr. Iddings 

 with a paper on the "Origin of quartz in basalt,":): in which, by a series of 

 analyses of quartz-bearing and quartzless basalts from New Mexico and 



* BulL U. S. rjcol. Smv(\v, No. 17, 1^85. 

 t Am. .Joiir. of Science, .laniiiny, 1HH7. 

 t //)(■(/., Sfptcml»or, 1H8K. 



