Igneous masses, when subjected to the same processes, give rise to rocks entirely similar to those made by the metamorphism of sediments, so that it is sometimes impossible to distinguish between them. The compression may have been applied while the magmas were still pasty, or long after they had cooled and solidified. Certain rocks, like the gneiss of Manhattan Island, are believed to have been formed both from the metamorphism of sediments and the injection of igneous material, and thus to have had a highly complex origin.

The Causes of Metamorphism have already been suggested in the preceding paragraphs, but it will be well to summarize them, though it should be borne in mind that the metamorphic processes are by no means completely understood.

1. Heat

Heat is evidently a very important factor of change, as is shown by the phenomena of contact metamorphism and by numerous experiments by which the process has been imitated successfully. In contact metamorphism the heat is derived from the igneous magmas, and in dynamic it is in part mechanically generated, in part due to the interior heat of the earth invading deeply buried masses.

2. Compression

Compression is believed to be the great agent of dynamic metamorphism, and the amount of the change depends upon the intensity of compression and the depth at which it operates. Hence the varying results, ranging from gentle folding, at one end of the series, through violent folding to complete reconstruction, crystallization, and foliation, at the other.

3. Moisture

Moisture is another potent agent of reconstruction. Superheated water under pressure is able to attack and dissolve the most refractory substances and to build them up into new combinations. Many minerals, such as the felspars, which have never been artificially crystallized by dry heat alone will crystallize readily in the presence of superheated water, and the water lowers the temperature necessary for metamorphism. Rocks which melt at 25000 F. dry heat, become pasty at 7500 F. in the presence of water. In contact metamorphism, steam is a very important factor of change, but other vapours and gases play an efficient part.

4. Pressure

Pressure, as distinguished from active compression, is a necessity for any extensive metamorphic action, whether contact or dynamic. It is the difference of pressure which is responsible for the different effects of surface flows of lava and of subterranean intrusions and which gives to depth its importance as a controlling factor. The dead-weight pressure of overlying rocks prevents the rapid escape of the mineralizing vapours, and, when sufficiently great, causes the rock to shear and "flow without fracture. Limestone heated at the pressure of the atmosphere, in a lime-kiln or an open vessel, becomes quicklime (CaO) through the expulsion of C02, but heated under pressure, so that the gas cannot escape, it crystallizes into marble. Such pressure, also, is an essential factor in dynamic metamorphism as a precondition in enabling the rock to behave more or less plastically under active compression and without shattering. Dynamic metamorphism must therefore take place at considerable depths below the surface.

It is believed by many geologists that metamorphism may proceed so far as completely to melt a sedimentary rock, producing a magma which is indistinguishable from a typically igneous one. Such extreme metamorphism has not been demonstrated for any considerable body of rocks, but may be true, nevertheless, and if so, we should then have the cycle of rock transformation complete, from igneous rock, through sedimentary and metamorphic, back to igneous. Be this as it may, certain metamorphic rocks do undoubtedly form a common meeting place for the sedimentary and igneous classes.