300 King’s Systematic Geology of the 40th Parallel. 
although it is possible that the temperature of fusion existed 
ut was prevented by the counteracting pressure from produc- 
ing liquefaction. The hypothesis is summarized thus: 
Conditions of metamorphism.—1. There is a horizon below 
the surface, at a depth which increases with the secular cooling 
of the globe, at which the heat and pressure are sufficient to 
produce the chemical activity needed to effect metamorphism. 
2. This horizon sinks deeper with the secular cooling. 3. So 
fusion must be near the surface. The observed rate of increase 
would indicate that at a depth of about fifty miles the tempera 
ture would produce fusion of rock if not prevented by the 
pressure. Now if by removal of the superincumbent material 
