In the present paper synthetic melts and natura! rocks have been comparedIn the present paper synthetic melts and natura! rocks have been comparedin order to flnd the laws governing the interrelation between the individual feidsparphases met with in rocks. Some of the preliminary results have been publishedelsewhere more than three years ago (Naturen 19 34, pp. 190-192) By comparingthe results of recent synthetic studies with new data obtained from natura! rocksthrough several years of gathering, some conclusions now seem so deflnite as tojustify publication, although many questions still await their explanation.