8 T. C. CHAMBERLIN 



that the large inequalities and asymmetries now observed are 

 largely due to later shiftings, distortions, and outgrowths of the 

 primitive elements. These then are the special subjects of study 

 in the third field of diastrophism. 



THE PARTICULAR OCCASION TOR THIS INQUIRY 



Now in a recent study of what could plausibly be assigned to 

 original irregularity in segmentation and what then remained to be 

 assigned to subsequent movements and unequal growths, I was led 

 to see, or to think I saw, evidence of a system of shiftings and of 

 unequal growths which marshaled themselves in a singularly 

 rational way as though they were due to systematic causes of a 

 general nature. The particular adjustments appeared to be such 

 as were directly implied by the configurations which the great 

 features now bear. By reasoning back from the present con- 

 figurations to the assigned primitive configurations, rather specific 

 amounts of shiftings and deformations, abetted by unequal out- 

 growths, seemed to be indicated. The amounts of these shiftings 

 were distinctly larger than the movements commonly assigned to 

 diastrophisms in the surficial fields. Because of this largeness the 

 question, How much shrinkage can reasonably be assigned the 

 earth during its whole history ? came up in a new and specific 

 form, and with especial piquancy by reason of unexpectedly exacting 

 demands. 



COMPARISON BETWEEN THE EARTH AND ITS NEIGHBORS 



In casting about for some independent means of estimating such 

 reasonable possibilities or even probabilities of shrinkage as there 

 might be under the later view of the constitution of the earth, a 

 comparison of our planet with its near neighbors, the moon, Venus, 

 and Mars, suggested itself, as also a comparison with an ideal earth 

 built of material of the average meteoritic type. 



The earth, Venus, Mars, and the moon form a little group of 

 closely related. bodies revolving in the inner part of the sphere of 

 control of the sun under very similar dynamic conditions. We 

 naturally think of them as widely deployed, but, taken all together, 

 the Httle group spans less than 3 per cent of the radial reach of 



