In the continental areas the outer 60 miles of the earth is herein referred to as the isostatic layer. This is subdivided from the surface downward as follows: a 10-mile granitic and sedimentary layer, a 20-mile intermediate layer of basic crystalline rocks, and a 30-mile layer of tachylyte. The uppermost and intermediate layers are generally referred to as the crust.

Below the isostatic layer the earth is cooling and shrinking. Stresses in the isostatic layer are piled up until the breaking point is reached when these stresses are relieved by deformation of the crust. Mountain ranges are folded up and geosynclines are downwarped. Approximate isostatic balance is maintained throughout this series of events.

Great masses of sediments will be deposited in the downwarps. A 2,000-foot basin will hold 12,000 feet of sediments because the deposited material is 56 as heavy as the material below that is replaced during periods of isostatic adjustment. The trough will be further deepened by rise of the level of base level. This is brought about by displacement of the sea water as the land masses are deposited in the ocean basins. Downfolding of the geosynclines will also occur as a result of geothermal changes. It is estimated that the rate of erosion exceeds the rate of thermal changes within the earth so that during a period of erosion the isogeotherms will be turned up. Downwarping will follow base-leveling merely as the isogeotherms return to normal. This may amount to as much as 1,000 feet and may be added to the downwarping brought about by crustal collapse and deepening resulting from the rise of the level of base level. In general about 9,000 more feet of sediments may be assigned to the basin through deepening brought on by thermal and base-level changes, making a total of 21,000 feet of beds.

Collapse of the crust at this time will fold this prism of sediments into a great mountain system. The beds must be downfolded about six times as much as they are upfolded in order to maintain isostatic balance. Compressive stresses will be dominant and the types of structures that will be formed are close folds, overturned folds, and thrust faults. Erosion of the mountain system will be accompanied by periodic vertical thrusts. These are effected as a result of isostatic adjustment. Compressive stresses will still be present but they will be secondary and brought on as only incidental to the vertical movements.

Final erosion of this mountain system will leave the isogeotherms turned up. As the isogeotherms return to normal the resulting shrinkage will form a basin right where the high mountain range once stood. Crustal collapse will deepen the downwarp already begun and the new series of sediments will be separated from the first by a very marked angular unconformity.

Another series of events may be started during the late stages of erosion. The viscosity of the subcrustal tachylyte layer may be reduced, by removal of overlying rock pressures and slight rise of temperature, to the point where crystals of olivine may begin to form in it. Once this process of fractional crystallization commences it will gain in momentum and a magma will be formed. Convection currents will stir the liquid body and it will work its way upward by selective fusion. When it reaches the zone of fracture it will be intruded and extruded. Repeated intrusions and extrusions will occur until the magma finally will be solidified. Cooling of the magmatic zone will cause shrinkage and downwarping, and this area once again will become the site of a geosyncline.

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