250 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



around the ocean basins, comes from beneath the latter. In order to 

 test this suggestion by an independent investigation, a study was made 

 of the conditions of fusion of basic and acid igneous rocks, and the con- 

 clusion is reached that basic rocks would melt under conditions of dry 

 melting while the acid rocks were yet very far from reaching a melting 

 temperature. Eruptions of basic rocks from deep-seated sources to the 

 surface would follow ; a disturbance of isostatic equilibrium would result, 

 and the heavy masses, seeking their appropriate level, would sink and form 

 depressions which would become ocean basins. The line of thought thus 

 suggests an origin of ocean basins and explains the occurrence of basic 

 eruptions in their margins. 



Introduction 



THESIS 



The lithosphere is characterized by a structure or structures imposed 

 upon it by the forces of gravitation, heat, and chemical attraction. An 

 analysis of the stresses resulting from imperfect equilibrium of heteroge- 

 neous masses indicates that the resulting structures, whose form is de- 

 termined by the orientation of foliation in metamorphic schists, are 

 disk-like bodies — that is, that the outer shell of the lithosphere has a 

 discoidal structure. 



ESSENTIAL CONCEPTS 



For the purpose of this discussion, the lithosphere is defined as that 

 rigid, elastic layer of the solid earth in which the elements are combined 

 to form minerals and minerals are aggregated to form crystalline rocks. 

 Two other states of rock are recognized, sedimentary and molten, but 

 they are regarded as incidental rather than as essential to the lithosphere. 



Crystalline rocks may be classified, according to their texture, as 

 granular and foliated. The granular texture originates by cooling of a 

 molten magma under balanced stresses in all directions. The foliated 

 texture develops when crystallization or recrystallization takes place in 

 the presence of unbalanced or non-uniform stresses. The nature and 

 distribution of stresses in the lithosphere must then determine the nature 

 and distribution of the crystalline masses of which it is composed. 



The lithosphere is heterogeneous as to density, and there- is a tendency 

 toward isostatic equilibrium between the heavier and lighter masses. The 

 attainment of equilibrium is, however, opposed by rigidity, and any 

 approach to equilibrium is more or less effectively canceled by erosion 

 and deposition. Equilibrium has, therefore, been least complete during 



