CLASSIFICATION OF IGNEOUS ROCKS. 905 



gradations between many kinds of rocks to which names have been assigned, 

 it is evident that it is impossible at the present time to trace out the stages 

 of alteration of the various rock species. All that is now possible is to indi- 

 cate how metamorphic igneous rocks may be handled in order to lead at 

 some future time to a systematic treatment similar to that applied in the 

 present treatise to the sedimentary rocks. 



A series of names has been proposed (pp. 776-784) to designate the 

 different kinds of alterations to which rocks are subjected, and their iisage 

 pointed out. But it seems desirable here to apply these usages to the igneous 

 rocks, even at the risk of repetition. If it is desirable only to express the 

 general fact of alteration, without indicating its character, the term u meta" 

 is prefixed. If the alterations have taken place under mass-static conditions, 

 so that textures and structures are preserved, the term "«j3o"is prefixed. If 

 the alterations have taken place under mass-mechanical conditions, so that 

 a slaty, schistose, or gneissose structure is produced, the terms schistic 

 or gneissic may be prefixed. If it is desired to emphasize the structure 

 rather than the kind of rock, the name of the rock may be placed before 

 the structural name, as gabbro-schist. Finally, the dominant metamorphic 

 minerals not implied by the name of the rock may be prefixed, as, for 

 instance, chlorite meta-dolerite. Another illustration is hornblende-dolerite. 

 This is the proper name for rocks which were originally dolerite but in 

 which the pyroxene has changed to amphibole. Such rocks occur exten- 

 sively in the Lake Superior region, where they have been generally called 

 diorite. But they do not ordinarily have the same chemical composition 

 as the diorites, in which the hornblende is original, nor is the feldspar 

 usually the same. If accompanying the alteration a secondary structure 

 is produced, the altered dolerite may become hornblende-schist-dolerite or 

 hornblende-dolerite-schist. 



Each of the igneous rocks has a certain range in chemical composition 

 and is composed of certain important minerals. The alterations to which 

 the constituent minerals of rocks are subject in the zones of katamorphism 

 and anamorphism and in the belts of weathering and cementation of the 

 former zone have been fully given (see Chapter V), and the variations in 

 the alterations in each of these zones and belts have been fully discussed. 

 (Chapters IV, VI, VII, and VIII.) Supposing the zone and conditions to 



