In contact metamorphism, thin sheets of molten rock produce less effect than thicker ones.  The strongest heat effects are naturally caused by bosses and regional intrusions, and the zone of change about them may be several miles in width.  In these changes heated waters and vapors from the masses of igneous rocks undoubtedly play a very important part.

Which will be more strongly altered, the rocks about a closed dike in which lava began to cool as soon as it filled the fissure, or the rocks about a dike which opened on the surface and through which the molten rock flowed for some time?

Taking into consideration the part played by heated waters, which will produce the most far-reaching metamorphism, dikes which cut across the bedding planes or intrusive sheets which are thrust between the strata?

Regional metamorphism.  Metamorphic rocks occur wide-spread in many regions, often hundreds of square miles in area, where such extensive changes cannot be accounted for by igneous intrusions.  Such are the dissected cores of lofty mountains, as the Alps, and the worn-down bases of ancient ranges, as in New England, large areas in the Piedmont Belt, and the Laurentian peneplain.

In these regions the rocks have yielded to immense pressure.  They have been folded, crumpled, and mashed, and even their minute grains, as one may see with a microscope, have often been puckered, broken, and crushed to powder.  It is to these mechanical movements and strains which the rocks have suffered in every part that we may attribute their metamorphism, and the degree to which they have been changed is in direct proportion to the degree to which they have been deformed and mashed.

Other factors, however, have played important parts.  Rock crushing develops heat, and allows a freer circulation of heated waters and vapors.  Thus chemical reactions are greatly quickened; minerals are dissolved and redeposited in new positions, or their chemical constituents may recombine in new minerals, entirely changing the nature of the rock, as when, for example, feldspar recrystallizes as quartz and mica.

Early stages of metamorphism are seen in slate.  Pressure has hardened the marine muds, the arkose, or the volcanic ash from which slates are derived, and has caused them to cleave by the rearrangement of their particles.

Under somewhat greater pressure, slate becomes phyllite, a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes.  The same pressure which has caused the rock to cleave has set free some of its mineral constituents along the cleavage planes to crystallize there as mica.

Foliation.  Under still stronger pressure the whole structure of the rock is altered.  The minerals of which it is composed, and the new minerals which develop by heat and pressure, arrange themselves along planes of cleavage or of shear in rudely parallel leaves, or folia.  Of this structure, called foliation, we may distinguish two types,—­a coarser feldspathic type, and a fine type in which other minerals than feldspar predominate.