Although it serves our convenience to refer the movements of the crust to the sea level as datum plane, it is understood that this level is by no means fixed. Changes in the ocean basins increase or reduce their capacity and thus lower or raise the level of the sea. But since these basins are connected, the effect of any change upon the water level is so distributed that it is far less noticeable than a corresponding change would be upon the land. CHAPTER XIII METAMORPHISM AND MINERAL VEINS Under the action of internal agencies rocks of all kinds may be rendered harder, more firmly cemented, and more crystalline. These processes are known as _metamorphism_, and the rocks affected, whether originally sedimentary or igneous, are called _metamorphic rocks_. We may contrast with metamorphism the action of external agencies in weathering, which render rocks less coherent by dissolving their soluble parts and breaking down their crystalline grains. =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 widespread 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.