624 JOHN JOHNSTON AND PAUL NIGGLI 
extended experimental investigation and a long time. In such 
investigation the first thing necessary is a definite conception of 
the general processes of rock metamorphism; this it has been the 
purpose of the authors to present in the foregoing pages. The 
choice of particular problems in this large field will doubtless be 
aided greatly by a study of natural mineral associations from the 
physical chemical standpoint, a study which at the same time will 
certainly provide us with information bearing directly on the 
problems at issue. 
WASHINGTON, D.C. 
July, 1913 
