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The literature on effects of high pressure has expanded enormously in recent years, making possible a complete revision of the section in the old edition. Attention has been confined to materials of geologic interest.

Most of the data in this section are in the form of lines of univariant equilibria projected into the pressure-temperature plane. Pressure effects on multivariant equilibria have been much less studied; a few cases are given in diagrams. Work on two important groups of minerals, the clays and zeolites, has been largely omitted. Many members of these groups are stable only at low temperatures, and difficulty in reaching chemical equilibrium is extreme. As a consequence conflicting results are to be found in the literature, and a certain amount of erroneous data must have been published.

In compiling the tables a critical selection of data has been attempted. Several reactions which have been studied have been omitted because it was not felt that the results were sufficiently reliable. In some cases results from one laboratory have been included in preference to those from another; results from two conflicting studies have both been included when no suitable basis for choosing between them could be found. Virtually all the data given here have been obtained from direct studies of phase equilibria at high pressures and temperatures. Thermodynamic calculations have been accepted when experimentally verified (diamond-graphite equilibrium) and in the case of the spinel-olivine transition in forsterite, which is still beyond the reach of direct experimentation.

Most of the data

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