88 WHITMAN CROSS 



bodies. That the above statements are true is abundantly 

 admitted by Professor Rosenbusch in the pages of his " Mikro- 

 skopische Physiographic der massigen Gesteine." 1 



The classes of igneous rocks established by Rosenbusch upon 

 the criterion of geological occurrence are not those of the geol- 

 ogist. But it is well understood by all familiar with the subject 

 that an assumed relation between geological occurrence and 

 structure of igneous rocks lies at the basis of Rosenbusch's 

 inconsequent definitions of the three classes under discussion. 

 In fact, the petrologist studying the genesis of igneous rock 

 structures knows that they result from a complicated set of 

 chemical and physical conditions attendant upon the consolida- 

 tion of molten magmas. These conditions are as yet only par- 

 tially understood. Pressure, absolute temperature, rate of cool- 

 ing, the chemical changes in the fluid residue owing to fractional 

 crystallization, the influence of so-called mineralizing agents, 

 and several other factors, are recognized, but their relative 

 importance is yet a matter of theory or hypothesis. A predom- 

 inating influence was not long ago assigned to pressure, meas- 

 ured by distance from the earth's surface. But it is now known 

 that that condition is in itself of little importance, within the 

 zone of the earth's crust of which we have definite knowledge. 

 It is also known that the conditions of consolidation are not con- 

 trolled by either geological form or place of occurrence, to an 

 extent capable of definite statement. The petrologist must rec- 

 ognize that while the typical granular structure is most common 

 in abysmal igneous masses it may develop and is often found 

 in the intrusive bodies of the intermediate zones of the crust and 

 in surface masses. Nor has size of the molten body a determin- 

 ing influence. Neither are the porphyritic or fluidal structures 

 dependent upon the geological form or place of occurrence of 

 rocks exhibiting them. 



Since the petrologist must inevitably apply the geological 



1 The natural limitations of the present discussion prevent an analysis of the con- 

 siderations which led the German master to propose such a classification, but a review 

 of the Rosenbusch system upon the principles here presented is now in preparation 

 and will soon appear in this Journal. 



