ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 41 



very great, but that as yet we can only make qualitative state- 

 ments in reference to its amount ; that quantitative statements 

 are objectionable because they imply a definiteness of knowledge 

 not warranted by the facts, and therefore stay the progress of 

 investigation. 



PART II. CAUSES OF CRUSTAL SHORTENING. 



Secular cooling— The first cause of contraction to be con- 

 sidered, and the only one ordinarily considered, is secular 

 cooling. The amount of such contraction has been variously 

 estimated. But the largest calculated amounts which the physi- 

 cists will allow have always been disappointingly small to the 

 geologists. 



Mallet, on the hypothesis that the earth was liquid and 

 had a mean temperature of 4000 F., has concluded that the 

 earth "between its period of liquidity and its present state has 

 shrunk in diameter by 189 miles at the least." 1 At that time, 

 according to Mallet, the earth would have a mean radius of 

 4053.3 instead of 3958.8 miles. The surfaces of these spheres 

 would be respectively about 206,457,000 and 196,942,000 square 

 miles, and thus the surficial contraction of the earth would be 

 about 9,515,000 square miles. 



Dutton, making his calculation on another basis, concludes 

 if the earth once had a nearly uniform temperature of 7000 

 F., that "if we were to assign thirty miles as the diminution 

 of the earth's mean radius since the first formation of a cooled 

 exterior, we should probably reach the utmost limit consistent 

 with Fourier's theorem." 2 Taking the average radius of the earth 

 as 3958.8 as before, the radius of the earth before contraction, 

 according to Dutton, could not be more than 3988.8. The sur- 

 face of this expanded earth would be about 199,938,000 square 

 miles, which gives a surficial contraction of about 2,996,000 

 square miles. 



1 Volcanic energy : an attempt to develop its true origin and cosmical relations, 

 by Robert Mallet : Trans. Roy. Soc, Vol. CLXIII, 1873, P- 20 5- 



2 A criticism upon the contractional hypothesis, by C. E. Dutton : Am. Jour. Sci,. 

 Vol. VIII, 1874, p. 121. 



