268 



FRANK D. ADAMS 



each case over 500 average diameters were measured instead of 

 100, which latter number Rosiwal considers to be sufficient. 

 The measurements were, however, confined to a small number of 

 thin sections, namely two in the case of No. i, and four in the 

 case of No. 2, it being considered advisable to use only sections 

 cut from the actual specimen from which the material for 

 analysis was taken. The results obtained were as follows : 



Feldspar . . . 

 Nepheline. . 

 Pyroxene . . . 

 Hornblende 



Eiotite 



Olivine 



Iron Ore . . . 

 Apatite 



63.77 per 

 6.12 ' 

 9.26 ' 

 8.06 



8-56 

 2.12 



64.06 per cent. 



6.16 

 13.60 



1.29 " 



4.07 



1.40 " 



8.10 



1.29 



99-97 



In the case of No. i the results are substantially the same as 

 the calculated mode except that there is about 3 per cent, more 

 pyroxene and a correspondingly smaller proportion of feldspar. 

 This relatively high proportion of pyroxene is unusual, the 

 examination of thin sections of the rock for various parts 

 of the mountain showing that, as has been stated above, and 

 as is shown also by the calculation of the ?node of this speci- 

 men, there is usually a preponderance of hornblende over 

 pyroxene. In the case of No. 2 the chief difference between 

 the values measured and the calculated mode lies in the rela- 

 tively higher proportion of feldspar and lower proportion of 

 nepheline in the former. In this rock, however, it is very diffi- 

 cult to distinguish the nepheline from the feldspar in every case. 

 These discrepancies indicate that in applying Rosiwal's method 

 to comparatively coarse-grained rocks such as these, especially 

 if there be any tendency to irregularity in composition a consid- 

 erable number of thin sections should be employed in order to 

 obtain a true average of the rock as a whole. 



For purposes of comparison the analysis of the essexite from 

 Shefford mountain (No. Ill) has been reduced to its normative 



