CRUST OF THE EARTH. It was formerly believed by scientists that the interior of our globe is in a state of fusion due to excessive heat, and they accordingly gave the name 'crust of the earth' to the external solid portion of the earth with which we are familiar. Modern in v•stigation has tended to show that the interior of the earth may not be liquid, as this term is ordinarily understood, and the tenn 'crust of the earth•' suggesting as it does the liquidity of the earth's interior, has fallen somewhat into dis favor among scientists. The term lithosphere, meaning 'rock sphere.' has been proposed as a substitute, but in popular nsage it has not dis placed the earlier term. The erust of the earth is composed of igneous and sedimentary rocks. The rocks occurring deepest below the surface, chiefly of the Archa.an age. are igneous. 'Resting on them, and forming the surface rocks for much of the globe, are sedimentary rocks, which have everywhere the same general geological succes sion, although varying widely in minor strati graphy and structure. The basal igneous rocks also appear at the surface over considerable areas, and other igneous rocks are associated with the sediments.
The rocks of the earth's crust are fractured and folded, the folding. involving an actual plas tic deformation or 'flowage' of the rock mass. It is probable that fractures are confined to the outer portion of the earth's crust, while at a considerable depth, perhaps 10,000 meters, the pressure is so great that fractures cannot be de veloped, and the rock is deformed by flowage. For convenience in discussing its structure, Van Hise has accordingly proposed a division of the earth's crust into (1 ) a deep-seated zone of flowage, where rocks are deformed by flowage, and where fractures cannot exist; (2) an upper zone of fracture, where the rocks are deformed by frac ture alone; and (3) an intermediate zone of fracture and flowage. By surface erosion rocks which have been deformed in the zone of flowage may ultimately reach the surface of the earth, and hence it is that we have side by side the effects of deformation in all zones. Consult Van Rise. "Principles of North American Pre-Cam brian Geology," in Sixteenth Annual Report Geological Surrey. pt. i. (Washington. 1890). See CLEAVAGE; ARCTLEAN SYSTEM FAULT.