PRINCIPLES UNDERLYING METAMORPHIC. PROCESSES 607 
points lie within a very much narrower zone. From this we can 
infer that the same will be found for the minerals when the requisite 
data are known. 
There is therefore a considerable body of evidence then which 
suggests that any permanent deformation of a crystalline dry 
aggregate is determined by an actual melting of some part of the 
material—a melting which occurs as soon as the local stress reaches 
the melting-pressure corresponding to the temperature of the 
material—with subsequent resolidification; this resolidification 
consists in the production of crystals of that form which appears 
S 
a” 
8 a | 
cA 
a 
FA eee HY 
d ak | 
oa | 
etn 
Hef | ee 
HARDNESS 
\ 
Ea vas 
1000° 
MELTING POINT 
Fic. 4.—Diagram to exhibit the relation between hardness (as ordinarily 
measured) and melting-point. Circles represent elements; crosses or lines represent 
minerals, lines being used where the tables give a value such as 5.5—6.5. 
most readily under the particular conditions of pressure (and there- 
fore not necessarily of the original form) and, so far as the argument 
is concerned, recrystallization need not be complete. So far as we 
are aware, there is no evidence which directly contradicts the above 
hypothesis; on the contrary this hypothesis enables us to correlate 
and interpret a large number of diverse and apparently contra- 
dictory observations." 
This type of compression accounts easily for regelation; the 
most common example is the regelation of the substance water 
which, as it happens, is exceptional in that its freezing-point is 
t See Johnston and Adams, Am. Jour. Sci., XX XV (1913), 205. 
