THE STRENGTH OF THE EARTH'S CRUST 431 



With long time even a minute force will cause even a very viscous 

 fluid to flow. Solids, on the other hand, possess elasticity of form, 

 and below the elastic limit can hold shearing stresses indefinitely. 

 Above it they may flow and in so doing exhibit plasticity. The 

 phenomenon differs from viscosity in that the force must rise to a 

 certain magnitude before any gHding between molecules begins. 

 The crust, then, is plastic but not viscous. 



Although the theoretical distinction between plasticity and vis- 

 cosity is clear, recognition must be given to conditions where the 

 two states merge. This is especially true for undercooled glasses. 

 A glass in its molecular organization is a liquid and yet it possesses 

 definite elastic moduli and elastic limit. From this standpoint of 

 elasticity the glass, therefore, is a solid. Upon rise of temperature 

 there is, however, no absorption of latent heat to mark a change of 

 state, the elastic limit gradually lowers, disappears for prolonged 

 stresses, and elastically, the substance passes by gradation from a 

 solid to a liquid. The existence of these transition cases should not, 

 however, be permitted to obscure the real distinctions between 

 solid and liquid. 



The crust yields as a plastic solid to forces which strain it 

 beyond its elastic limit. But the solid fiowage which this imphes 

 may be either by distortion of crystals or by recrystallization. 

 The first is famiHar for rapidly apphed forces, requires compara- 

 tively great stress, and corresponds to the usual conception of 

 plasticity. The crystalhne rocks make us famihar, however, with 

 the idea of mass plasticity by recrystallization. This is plasticity, 

 but in a somewhat different sense from that which is usually con- 

 veyed by the term. 



The degree of elasticity which a substance may exhibit is a 

 different property from the elastic limit. A bar of wrought iron 

 one square inch in cross-section will be elongated one part in 28,000,- 

 000 by a tensile stress of one pound. A similar bar of glass would be 

 elongated one part in 10,500,000, more than twice as much. These 

 ratios measure the degree of elasticity under tensile or compressive 

 stresses and differ for each substance. The figure is known as 

 Young's modulus of elasticity. A substance may be highly elastic, 

 that is, have a high modulus of elasticity, as cast iron, or glass, 



