CONTACT METAMORPHISM. Rocks beneath a lava flow or in contact with igneous intrusions are found to be metamorphosed to various degrees by the heat of the cooling mass. The adjacent strata may be changed only in color, hardness, and texture. Thus, next to a dike, bituminous coal may be baked to coke or anthracite, and chalk and limestone to crystalline marble. Sandstone may be converted into quartzite, and shale into ARGILLITE, a compact, massive clay rock. New minerals may also be developed. In sedimentary rocks there may be produced crystals of mica and of GARNET (a mineral as hard as quartz, commonly occurring in red, twelve-sided crystals). Where the changes are most profound, rocks may be wholly made over in structure and mineral composition. 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.