MOVEMENTS AND DEFORMATIONS 365 



earth would have a dynamic effect similar to that of extrusions, 

 so far as the outer portion of the earth is concerned, and the amount 

 of intrusive rock is probably greater than the amount of extrusive; 

 far greater if the views of vulcanism here entertained are correct. 



Rotational possibilities. The oblateness of the present earth 

 is accommodated to its present rate of rotation. It is assumed 

 that such accommodation has always obtained, and that if the 

 rotation has changed, the form of the earth has changed also. The 

 more oblate the spheroid, the larger its surface shell and the less 

 the total force of gravity. If the earth's rotation has diminished, 

 its crust must have shrunk, because the form of the earth became 

 less spheroidal, and the increase of gravity increased its density. 

 Recent studies imply that the effect of this was probably small. 



Sphericity as a factor in deformation. It is obvious that if the 

 earth shrinks, its crust must become too large for the reduced 

 volume, and must be compressed or distorted to fit the shrunken 

 whole. If the radius shortens 5 miles, each great circle must on 

 the average be compressed, wrinkled, or otherwise distorted to the 

 extent of about 31 (5 x 6.28) miles. If the mountain foldings on 

 any great circle, taken together, show a shortening of 100 miles, 

 the corresponding radial shortening is about 16 miles. A segment 

 3,000 miles across (about -J- of a circumference of the earth), such as 

 the bottom of the North Atlantic basin, sinking three miles (about 

 the full depth of the basin), would give a lateral thrust of about 

 2.3 miles, a little over a mile on each side, a trivial amount com- 

 pared with the foldings on the adjacent continental borders. 



Each portion of the crust of the earth is ideally an arch or dome. 

 When broad areas like continents are considered, it is the dome 

 rather than the arch that is involved, and in this the thrust is 

 ideally toward all parts of its periphery. A dome corresponding to 

 the sphericity of the earth, formed of firm crystalline rock of the 

 high crushing strength of 25,000 pounds to the square inch, and hav- 

 ing a weight of 180 pounds to the cubic foot, would, if unsupported 

 below, sustain only ^-^ of its own weight. 1 This result is essentially 

 independent of the extent of the dome, and also of its thickness, 

 provided the former is continental, and the latter does not exceed 



1 Calculations by Hoskins. 



