152 WAEREN. 



ning at all. The ^^Titer has studied a number of rocks in which or- 

 thoclase was described as being present only to find, on careful study, 

 that the alleged orthoclase was really microcline. 



In comparing the feldspars of the pegmatites with those of the 

 granites with which they are connected genetically, or of (Hfl'erent 

 granites, it is of course impossible, with the rather meager data now at 

 our command, to estimate definitely the effect of constituents other 

 than the feldspars which are present in the respective magmas on the 

 equilibrium relations existing between the feldspars. The effect of 

 varying pressures is also largely of unknown magnitude. In the case 

 of granite magmas, relatively poor in anorthite and mafic minerals, 

 the principle differences in chemical composition as compared with the 

 pegmatitic magmas which are derived from them, are generally con- 

 sidered to be the greater amounts of water or water vapor and other 

 of the more volatile constituents — mineralizers — of the pegmatitic 

 magmas. The presence of greater amounts of water would probably 

 have the effect of collecting different relative amounts of the two 

 feldspars in the one as compared with the other. Probably with the 

 greater excess of water in the pegmatitic magma would go a relatively 

 greater amount of the more soluble (under those conditions) feldspar 

 molecules. That the excess water would have any great effect in 

 modifying the general relations of the two feldspars during crystalliza- 

 tion in the pegmatites as compared with the granites, the writer very 

 much doubts. One effect of larger amounts of water etc. would very 

 probably be to facilitate any adjustment of equilibrium that might be 

 imminent, particularly where the adjustment had to take place in the 

 solid state. In the pegmatites the possible presence of more active 

 residual liquors in contact with the crystals might conceivably have 

 the effect of carrying the unmixing somewhat further in the direction 

 of extracting more of the more soluble phase. As this is probably the 

 albite, it may be, that in the pegmatitic feldspars, such as those here 

 studied, the microcline phase contains a somewhat lower percentage 

 of the albite than is the case for corresponding feldspar in the granites. 

 The differences in the two cases are, in the wi-iter's opinion, likely to be 

 rather slight at least so far as the main crystallizations of pegmatites 

 and granites in general are concerned. The residual liquors in all 

 cases would carry some feldspar in solution along with other materials, 

 and in special cases, perhaps considerable amounts. These liquors 

 might segregate centrally or otherwise, or migrate, and in any case 

 would finally deposite their dissolved material as separate crystalliza- 

 tions, distinctive in character, of albite and microcline or, under cer- 



