MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 561 



So also it appears from the diagram that there was practically no con- 

 traction below 160 miles up to the end of the 100,,000,000-year period, 

 because cooling had not yet reached that depth. Between these two 

 non-contracting horizons the greatest rate of contraction at the close 

 of the 100,000,000-year period lay about 60 miles below the surface. 

 The contraction of this middle zone, while the outermost shell and the 

 interior body remained constant, is held to have developed a state of 

 horizontal thrust in the outer shell, because this shell, being too large for 

 the shrinking subcrust, tended to settle, and to crowd upon itself hori- 

 zontally. The wrinkling and other modes of deformation of the outer 

 part of the earth are referred, under this view, to the thrust so de- 

 veloped. This is the view which has been most generally accepted. 



Level of no stress. — As the outer shell is thus held to be in a state of 

 thrust while the zone below is in a state of shrinkage, there must be, 

 between these two zones, a level of no stress, where there is neither compres- 

 sion nor stretching. Above this level, the thrust increases to the surface, 

 and below it, the stretching increases to the depth of most rapid change 

 of temperature, below which it decreases and finally vanishes at the 

 lower limit of temperature change. In the earliest stages of cooling, 

 the level of no stress must have been near the surface, and must have 

 descended gradually as the cooling proceeded. The depth of this level 

 has been repeatedly computed on the basis of assumed times and rates 

 of cooling. Fisher, assuming the temperature of solidification to have 

 been 4000° Fahr. and the period of cooling 33,000,000 years, computed 

 its depth at only yV of a mile below the surface.^ T. Mellard Reade, 

 with somewhat different assumptions, placed it at 2 miles after 100,000,- 

 000 years of cooling.^ Davison (1897) placed it at 2.17 miles,^ and 

 G. H. Darwin at 2 miles after the same period.^ In a later computation, 

 based on the assumption that the coefficient of dilatation increases 

 with the temperature, Davison placed the level of no stress at 7.79 miles, 

 and stated that if the coefficient of conductivity and the initial heat 

 also increased downwards, the zone would lie still deeper. To suppose 

 the initial heat to increase downwards, however, is to abandon the 

 hypothesis we are now considering. These computations seem to show 

 that, at the very utmost, the level of no stress, under this hypothesis, 



* Physics of the Earth's Crust, Fisher, p. 95. 



2 Origin of Mountain Ranges, T. Mellard Reade, p. 125. 



8 Phil Trans. Roy. Soc, Vol. 178, pp. 231-49. 



