484 JOHN JOHNSTON AND PAUL NIGGLI 
their effectiveness and limitations, in order that effects may not be 
attributed to causes which are incompetent to account for them. 
The present paper is an endeavor to discuss the validity, the limi- 
tations, and the relative importance of the general principles 
involved; our aim has been, in bringing them definitely to the 
attention of geologists, to show that it is no simple matter to apply 
these principles, simple in themselves, to the very complicated 
systems encountered under geological conditions, and that very 
careful reasoning is required if correct conclusions are to be reached. 
We wish to emphasize especially the distinction which must be 
made between uniform pressure and non-uniform compression or 
stress. The importance of uniform pressure, and the magnitude 
of the effects producible by it, have frequently been overestimated, 
while on the other hand insufficient account has been taken of the 
comparatively much greater effects which may result from the 
action of non-uniform compression. 
We shall not go into the various kinds of metamorphism— 
regional-, contact-, dynamo-metamorphism, etc., for in all cases the 
final effect is determined by the same factors, namely, temperature, 
pressure, stress, chemical composition,’ and speed of reaction. One 
factor may be predominant in one kind of metamorphic process, 
another factor in a second; but by combination of the above 
factors, all of the observed effects attributed to metamorphism may 
be accounted for. 
In the following pages we present a discussion of the most 
important general principles involved, especially of those principles 
which have not always been applied correctly by those who 
have made use of them. We have, for the sake of clearness, 
divided up the subject-matter under a number of headings, to 
which we have, however, deemed it inadvisable to adhere strictly 
throughout; these are intended to be read only in the light of the 
general context, as it was impracticable to insert always the qualify- 
ing phrases required to render the statements strictly accurate. 
Moreover, this paper does not pretend to completeness, either from 
the chemical or from the petrologic side; it aims to treat the various 
' By this is to be understood the gross chemical composition of the whole system 
at the period of metamorphism, and not the chemical composition of therock-massnow. 
