PETROGRAPHY. 337 



ture of the eruptive rocks is dependent upon the conditions of 

 their geological occurrence, and classified them accordingly in 

 three chief groups : deep-seated or " plutonic " rocks, intru- 

 sive or "dyke" rocks, and eruptive flows or "sheets." This 

 new standpoint assumed by Rosenbusch re-acted upon the 

 whole newer development of petrography. By subordinating 

 in his new system all considerations of the chemical and 

 mineralogical composition, and the geological age, to the 

 mode of occurrence of eruptive rocks in nature, Rosenbusch 

 removed as it were the final judgment of petrography from 

 the laboratory to the field. The petrographer was made to 

 feel that the microscope and chemical re-agents were to be 

 regarded as aids to field observations, but that systematic 

 interest was to be concentrated upon the problems dealing 

 with rock-structure in its relation to particular conditions of 

 stratigraphical occurrence. In this direction original research 

 seemed to give most promise of enlightenment in the imme- 

 diate future. 



Rosenbusch introduced a number of new descriptive terms, 

 e.g., holocrystalline, hemicrystalline, hypidiomorphic, panidio- 

 morphic, etc., for the purpose of defining all structural modifi- 

 cations with scientific accuracy. According to Rosenbusch, 

 the deep-seated eruptive rocks are all distinguished by holo- 

 crystalline and hypidiomorphic granular structure. They have 

 originated at great depths of the crust by slow processes of 

 cooling and consolidation. He divides them into sub-groups 

 which are based upon the presence and relative amount of 

 quartz and felspar; in this respect, therefore, Rosenbusch 

 adopted the system of MM. Fouque and Michel-Levy. 



Rosenbusch includes in his group of intrusive rocks those 

 eruptive masses which occur in the form of typical dykes, yet 

 are to be regarded only as particular facies of deep-seated 

 eruptive rocks, and may probably be associated with the latter 

 in their genesis and their distribution in the crust. The intru- 

 sive group is sub-divided into three series a granitic, a 

 syenitic, and a dioritic, whose characteristic types of structure 

 are quite independent of their mineralogical composition. 



Porphyritic structure is said by Rosenbusch to be character- 

 istic of eruptive sheets; the constituents belong to at least 

 two successive generations. He thinks it probable that the 

 older constituents represented by the larger crystalline ele- 

 ments are intra telluric in origin, and may have formed at 



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