The causes of deformation.  As the earth’s interior, or nucleus, is highly heated it must be constantly though slowly losing its heat by conduction through the crust and into space; and since the nucleus is cooling it must also be contracting.  The nucleus has contracted also because of the extrusion of molten matter, the loss of constituent gases given off in volcanic eruptions, and (still more important) the compression and consolidation of its material under gravity.  As the nucleus contracts, it tends to draw away from the cooled and solid crust, and the latter settles, adapting itself to the shrinking nucleus much as the skin of a withering apple wrinkles down upon the shrunken fruit.  The unsupported weight of the spherical crust develops enormous tangential pressures, similar to the stresses of an arch or dome, and when these lateral thrusts accumulate beyond the power of resistance the solid rock is warped and folded and broken.

Since the planet attained its present mass it has thus been lessening in volume.  Notwithstanding local and relative upheavals the earth’s surface on the whole has drawn nearer and nearer to the center.  The portions of the lithosphere which have been carried down the farthest have received the waters of the oceans, while those portions which have been carried down the least have emerged as continents.

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.