482 JOHN JOHNSTON AND PAUL NIGGLI 
Part II 
THE PHASE RULE 
Qualifications in Relation to Metamorphic Systems 
Illustrations: System CaO-Al.0,-SiO, 
System CaCl,-MgClL-H.,O 
Consideration of an Ideal Case 
GENERAL EFFECTS OF NON-UNIFORM COMPRESSION 
Effect on Solid Phase Alone 
Effect on Systems Solid-Liquid 
Derivation of the Formula 
Effect on Solubility 
Application to Metamorphic Processes 
Effect on Reactions with Vapor Phase 
METAMORPHISM WITH ADDITION OF MATERIAL 
CONCLUSION 
PART I 
INTRODUCTION 
The term metamorphism in its widest sense includes all the 
alterations which occur in solid rock; it therefore comprises those 
processes which change the composition (mineral or chemical), 
structure, and texture of rocks after they have been formed, no 
matter what may have been their origin—by solidification from 
magma, consolidation of chemical precipitates, or mechanical 
deposit. But we, following Grubenmann, shall limit this somewhat 
and consider as typical cases of metamorphism only those in which 
the effects produced by the alterations determine completely the 
character of the rock mass. 
An essential characteristic of metamorphic alterations is that 
the rock as a whole remain solid during the process. This condi- 
tion does not hold whenever the original rock has at any instant 
been wholly melted; such a case is merely a solidification under 
special conditions from a magma, and the rock formed in this way 
is indistinguishable from an eruptive rock. When once a rock has 
been completely melted, we can say nothing of its particular his- 
tory previous to that time; in especial we cannot know whether 
any of the components of the rock have existed previously as solids. 
