THE GEOLOGICAL EVIDENCES. 47 



of increase of internal density was taken as perhaps the best expression 

 of this factor and as being in fair accord with astronomical data. Pro- 

 fessor Slichter extended the computation to other constants of the earth 

 than those requisite for this inquiry and these give to his paper a value 

 quite independent of its application to the present problem. His paper 

 and table will be found on pages 61-67 of this volume. 



Column 7 of this table (page 67) shows that ten periods of rotation 

 have been selected, ranging from 3.82 hours to the present period. Darwin's 

 hypothesis * leaves unassigned the precise period of rotation when separa- 

 tion took place, but from an inspection of the configuration of the spheroid 

 at the rotation-period, 3.82 hours, and of the gravity in different parts of 

 the spheroid at that stage, it seems safest to assume that a rotational 

 period less than 3.82 hours would be necessary to cause fission. It seems 

 best also to assume that at the 3.82-hour stage the earth was solid on the 

 exterior, whatever may have been its internal condition. If this shall not 

 seem so to any one, the arguments based upon the data of this rotational 

 period can easily be shifted to the numerical values of the next period of 

 4.03 hours, or to any of the later periods given in the table. 



From column 11 it will be seen that the equatorial circumference at 

 the rotation-period, 3.82 hours, was 1,131 miles greater than it is at present, 

 while the meridional circumference was 495 miles less. In changing to 

 the present form, the tract immediately under the equator must have 

 become shorter by 1,131 miles. The tracts under the parallels adjacent to 

 the equator north and south would have become shorter by less amounts, 

 those still farther away by still less amounts, until a little beyond 30^ 

 latitude, north and south, parallels are reached under which the crust would 

 have theoretically remained unchanged so far as this immediate factor is 

 concerned. These are the latitudes of mean radius for each stage of rota- 

 tion and are shown in column 9. It will be noted that these shift from 

 lat. 33° 20' to 35° 13' in the course of the series, but it is sufficient for our 

 purpose to speak of the neutral zone as lying at 35° latitude, north and south 

 respectively. The equatorial belt between these parallels, 70° in width 

 roundly speaking, would therefore, by the postulated change, have become 

 shorter along its central line by 1,131 miles, since the rotation-period of 

 3.82 hours. On its borders it would have suffered no change, and between 

 the borders and the central line it would have suffered a graded series of 

 shortenings. 



That portion of the meridional circumference which lay within the 

 equatorial belt should have been shortened in the course of the change 

 from the rotation-period of 3.82 hours to that of the present, but the whole 

 meridional circumference should have been lengthened 495 miles. It 

 is obvious, therefore, that the areas north and south of the neutral zones 

 must have become extended meridionally 495 miles plus the amount of the 

 contraction in the equatorial zone, the precise value of which is unimpor- 

 tant here. It will be convenient to call these areas of expansion polar caps, 

 though they reach down to about 35° latitude. In the course of the change 

 named, the surface at the poles should have been raised and the curvature 



' The Tides, p. 360. 



