PETROGRAPHY. 329 



well as the British student is to be congratulated ui)on its appearance. 

 Other new works in book form, but which the present writer not hav- 

 ing seen can speak of only by title, are noted in the bibliography. 



ADVANCE WORK. 



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

 years before, has been purely descriptive of the mode of occurrence, 

 structure, mineral and chemical com[)osition of rocks, yet the subject of 

 the origin of rocks and the causes of their structural variability have 

 been by no means ignored, as will be noticed. 



THE PHYSICAL AND CHEMICAL CONDITIONS OF CRYSTALLIZATION. 



For many years there has been a growing feeling among geologists 

 that the importance attached, particularly amoug German authorities, 

 to geological age as a criterion in rock classification was greatly over- 

 estimated, and the fact that neither mineral composition nor structure 

 are necessarily dependent upon age is now very generally conceded. 

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

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

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

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

 among the more recent that the effusive portions have escaped erosion 

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

 known researches of Messrs. Hague and Iddings upon the rocks of the 

 Comstock Lode. Nevada,* showing that the " degreeof crystallization de- 

 veloped in igneous rocks is mainly dependent upon the conditions of heat 

 and pressure under which the mass has cooled and is independent of 

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

 seems now a well-established tact that under similar circumstances crys- 

 tallization and structure may be the same regardless of geological age. 

 More recent discoveries and studies in this same general line have beeu 

 productive of very interesting results. In a paper entitled " On the lat- 

 est volcanic eruption in ( 'alifornia and its peculiar lava,"t Mr. J. ^. Dil- 

 ler has described a very interesting type of rock, evidently a true ba- 

 salt, but uni(iuein carrying primary pori)hyritic quartz, associated with 

 olivine, a condition of alfairs ordinarily considered on chemical grounds 

 as not likely to occur. The peculiarity is explained on the sui)[)osition 

 that the quartz was the. first mineral to separate out from the nmgma, 

 and its crystallization took place under great pressure at such depth 

 and under such conditions of physical and chemical equilibrium as are 

 as yet largely conjectural. Mr. Diller has been followed by Mr. hhlings 

 with a pai)er on the "Origin of (juartz in basalt," J in which, by a series of 

 analysesof quartz-bearing and (juartzless basalts from New Mexico and 



"* Bull. II. 8. Cic^ol. Smvev, No. 17, 1H85. 

 t Am. Jour, of Science, .January, 1887. 

 t Ihiii, ScptciiilxT, 1^8):'. 



