368 



CARNEGIE INSTITUTION OF WASHINGTON. 



density from the smallest to the largest, they were studied as though 

 they represented the stages of growth of a single body. For this 

 purpose 1 per cent of the earth-mass was taken as the unit of increase, 

 and an ideal parity-earth built of meteorites taken as having a mean 

 density of 3.69 (Farrington's determination) as a standard of com- 

 parison. It is to be noted that much porosity probably prevailed in 

 the outer zone of the bodies of little or no atmosphere, while the 

 hydrospheres of Venus and the earth caused the filling of pores by in- 

 wash and cementation. Table 2 gives the results of the comparison 

 in this form. 



Table 2. — Comparison of actual bodies with corresponding ideal 

 parity bodies built of meteorites. 



Body. 



Moon 

 Mars. 

 Venus 

 Earth. 



Mass units 



(unit=l p. ct. 



earth). 



p. ct. 

 1.22 



10.65 



80.70(?) 

 100.00 



Relations, 



actual to 



parity 



body. 



miles. 

 -28.7 

 - 2.0 

 + 4.5 

 + 5.7 



It appears from this table that, on the assumption that the whole 

 work was compressive and making no allowance for the lighter ma,tter 

 of the large bodies, the moon, built up as it actually was, failed to 

 compress itself to the assumed meteorite standard by 28.7 miles per 

 unit of mass-growth, Mars by 2.0 miles per unit, while Venus compressed 

 itself beyond the meteorite standard to the extent of 4.5 miles per unit, 

 and the earth by 5.7 miles per unit. The trustworthiness of the 

 meteorite standard does not affect the real relations of the four bodies; 

 the standard is only a convenience in making the comparison. Sub- 

 sequent studies favor a lower rather than a higher mean density for 

 meteorites. If these four bodies fairly represent four normal states 

 of growth in a single body, their significance is highly important, for 

 they imply that the diastrophism which gave rise to the high density 

 and profound deformation of the earth took place, either actually or 

 potentially, during its formation and chiefly in the latest stages. 

 Because of this radical importance, it was felt to be imperative to 

 examine with care all related factors or assumptions as fully as prac- 

 ticable to see if in any essential way they affected the significance of 

 these conclusions. In making this examination, special attention was 

 given to the dynamic conditions that controlled planetary evolution 

 in the region of the earth under the hypotheses postulated, particularly 

 the relations of the formative processes to the spheres of control that 

 limited and governed them. 



