PERTHITIC FELDSPARS. 141 



duced by the error in the estimate of minute albite lamellae contained 

 in the microcline, and there is an even chance, at least, that it may off- 

 set that error. It is necessary to assume that the amount of potassic 

 feldspar dissolved in the albites of the other specimens is the same as 

 in the one analyzed, and it was also assumed that the lime shown by 

 the analyses, is all present in solid solution in the albite as the anor- 

 thite molecule. There appears to be very little likelihood of these 

 two assumptions being far wrong. Making the assumptions noted 

 above, the computation, from the data collected in Tables I and II, of 

 the chemical compositions of the two mixed crystals which enter into 

 the perthitic intergrowths is a simple matter. The results are given, 

 culculatcd to 100%, in Table III, in which are also repeated in an 

 abbreviated form a part of the data of Table II. Taking into consider- 

 ation the various sources of error it is thought that the total error is 

 certainly not greater than 2.0 units in the final figures for the mixed- 

 crystal phases and is probably less. 



Discussion of Results. — Reference to Table III will show that there 

 is quite a jump from No. 1, the feldspar from Perth, Ontario, with 

 52.3 per cent of the microcline member to No. 8, the next nearest, with 

 72.3 per cent. The remaining feldspars show a distribution of values 

 from 72.3 per cent to 86.2 per cent of microcline. Nos. 4, 5 and 6 

 are, however, quite close together, and admitting that the percentages 

 given may be 1 per cent too high or too low, may be said to be practi- 

 cally identical so far as the relative amounts of the two members are 

 concerned. A preliminary study of several other feldspars from other 

 localities leads the writer to believe that these too would, if measured, 

 fall within the range above given. As the feldspars studied are be- 

 lieved to be representative of granite pegmatite perthites in general, 

 it is perhaps safe to say that such feldspars will most often be found to 

 contain from 70 per cent to 87 per cent, approximately, of the micro- 

 cline phase. 



The amounts of the orthoclase and albite (plus anorthite) molecules 

 as computed from the chemical analyses also show a considerable 

 difference between No. 1 (Or. 51.9 per cent) and the next nearest, 

 No. 7 (Or. 67.2 per cent). The rest lie nearer together. Omitting 

 No. 1, the range of the remaining is from Or. 67.2 per cent to Or. 80.6 

 per cent, or 13.4 per cent. 



It will be seen that the amount of KAlSiaOs in the microcline falls 

 between 93.0 per cent and 88.5 per cent (±2.0). The variation appears 

 to be greater than the estimated error and probably represents real 

 differences of composition. The amount of Ab+An ranges between 



