THE STRENGTH OF THE EARTH'S CRUST 659 



sphere, even though it includes from time to time molten bodies, 

 the constituents of the pyrosphere. 



The theory of isostasy shows that below the lithosphere there 

 exists in contradistinction a thick earth-shell marked by a capacity 

 to yield readily to long-enduring strains of limited magnitude. 

 But if such a zone exists it must exercise a fundamental control 

 in terrestrial mechanics, in deformations of both vertical and tan- 

 gential nature. It is a real zone between the lithosphere above 

 and the centrosphere below, both of which possess the strength to 

 bear, without yielding, large and long-enduring strains. Its reality 

 is not lessened because it blends on the limits into these neighbor- 

 ing spheres, nor because its limits will vary to some degree with the 

 nature of the stresses brought upon it and to a large degree by 

 the awakening and ascent of regional igneous activity. To give 

 proper emphasis and avoid the repetition of descriptive clauses it 

 needs a distinctive name. It may be the generating zone of the 

 pyrosphere; it may be a sphere of unstable state, but this to a 

 larger extent is hypothesis and the reason for choosing a name 

 rests upon the definite part it seems to play in crustal dynamics. 

 Its comparative weakness is in that connection its distinctive 

 feature. It may then be called the sphere of weakness — the 

 astheno sphere, and its position among the successive shells which 

 make up the body of the earth is as follows: 



The atmosphere 



Including the biosphere 

 The hydrosphere 



The hthosphere 



Including the pyrosphere 

 The asthenosphere J 



The centrosphere, or barysphere 



Each has played its fundamental part in the development of 

 earth-history. 



STRESS-DIFFERENCES BETWEEN CONTIGUOUS COLUMNS OF THE CRUST 



Stresses under conditions of isostatic equilibrium. — -The conti- 

 nental platforms slope down into the ocean basins at grades which 

 range mostly from one in ten to one in thirty. Some of the great 



