266 CARNEGIE INSTITUTION 



Since experimental data relative to the compressibility of rock 

 must, at the best, always be limited to a rather narrow range, it is 

 important to make such use as is possible of the geological evidence 

 relative to the actual compression that has taken place. For the 

 statistical data bearing on this we must look to geological measure- 

 ments of the extent of the deformations that have been suffered by 

 the earth. In the interpretation of these the assignable sources of 

 the compressive agencies and their modes of action are important. 

 As one step in this direction, an attempt has been made to test the 

 validity of the suggestion, growing out of the researches of G. H. 

 Darwin, that a change in the rate of rotation of the earth, due to 

 tidal action, has reduced the volume of the earth by compression aris- 

 ing from increased gravity, and has also reduced the surface area by 

 change of form, in addition to the reduction due to lessened volume. 

 As Professor Slichter had previously made computations bearing on 

 this subject, his cooperation was sought, and generously given. As 

 a convenient approximation, a homogeneous earth had been assumed 

 in the previous computations. For a closer approximation, he re- 

 computed the reduction in surface area on the basis of an earth 

 whose interior density changed according to the Laplacian law. For 

 a broader application, the computations were made to include a series 

 of fourteen rotation periods, ranging from 3.82 hours to 23.934 

 hours. The following factors were computed for each period : polar 

 radius, equatorial radius, ellipticity, equatorial attraction, polar at- 

 traction, centripetal acceleration at the equator, latitude of mean 

 radius, equatorial contraction (in percentage and in miles), and 

 meridional contraction (in percentage and in miles). 



It will be seen that these data cover a multitude of special cases 

 and may be used in almost any case liable to arise. The mode of 

 application and the general tenor of the results may be illustrated 

 as follows : With a change of rotation from 3.82 hours to the pres- 

 ent rate, the equatorial belt must shorten 1,131 miles, while the 

 meridional circles must lengthen 495 miles. If the crust shortening 

 involved in the formation of the Alps be taken at 75 miles, regarded 

 as a very ample if not excessive estimate, the 1,131 miles of 

 equatorial shortening would be sufficient to form 15 mountain ranges 

 of Alpine magnitude. These should run across the equatorial belt 

 and die away at the latitude of mean radius, 32 22'. The con- 

 temporaneous high latitude tension would be sufficient to cause the 

 crust to gap or to stretch in the polar arcs more than 200 miles. 

 The coordinate geological inquiry shows no such remarkable distri- 



