EXTENT OF METASOMATISM. 641 



sedimentary and igneous rocks may go far toward or quite to completion 

 under mass-static conditions is so well known that the fact need not here 

 be emphasized. One of the best illustrative American localities is that of 

 the Keweenawan series of the Lake Superior region. As shown by 

 Pumpelly" and Irving, 6 the more porous lavas of this series have in many 

 cases largely recrystallized. The less porous ones show extensive alterations. 

 In various regions even great dense igneous masses have been profoundly 

 affected or completely recrystallized throughout by metasomatic change. 



However, as subsequently seen, the rocks which have recrystallized 

 under mass-static conditions are easily discriminated from those which have 

 been recrystallized under mass-mechanical conditions. 



CONDITIONS FAVORABLE TO METASOMATISM. 



Since the changes are produced mainly through the medium of water, 

 the presence of a considerable amount of water, and especially conditions 

 favorable to its circulation, are very favorable to metasomatic changes. It 

 follows that in proportion as the rocks are porous metasomatism is likelv to 

 be rapid. But if there be sufficient time, in order to produce profound 

 changes, there is no necessity for circulation of ground waters beyond 

 that necessary to provide water for the development of the hydrated 

 minerals which form in the belt of cementation so as to keep intact the 

 minute amount of water in the small openings. If these conditions obtain, 

 a very small amount of water may be the medium through which the rocks 

 are completely altered and recrystallized. Even the subcapillary openings 

 may be penetrated by solutions and the unstable minerals transformed 

 throughout. But in dense rocks under mass-static conditions, where the 

 spaces bearing water are all subcapillary, the changes are exceedinglv 

 slow. Even in the pre-Cambrian rocks, in the larger dense masses, such 

 readily alterable minerals as nepheline and olivine are found. Olivine 

 occurs extensively in the pre-Cambrian rocks of the Lake Superior region. 

 Throughout extensive - masses in this region the less alterable minerals, 

 augite and basic feldspar, are apparently almost perfectly fresh. The 



« Pumpelly, Raphael., Metasomatic development of the copper-bearing rocks of Lake Superior: 

 Proc. Am. Acad. Arts and Sci., vol. 13, 1878, pp. 253-309. 



6 Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. S. Geol. Survey, vol. 5, 1883, 

 pp. 87-91. 



MON XLVII — 04 41 



