F. L. Ransome — Apatitic Mineite. 341 



the albite molecule is present in the feldspar. It is, therefore, 

 to be concluded that the actual development of the abundant 

 biotite in the rock leaves available sufficient silica to form 

 albite rather than nephelite. A rough calculation of the 

 mode, in which the pyroxene is figured as ideal diopside and 

 the biotite as one having the ratios 



MffO FeO _ Si0 2 „ 



eJs= 4 ' k^t 1 ' and k75 =6 ' 



although not accurate enough to give the mode of the rock, 

 shows that, when biotite is allowed for, the readjustment of 

 the constituents locks up sufficient potash and disengages 

 enough silica to raise the remaining lenads to feldspar. 



