PENETRATION OF BASAL DIASTROPHISM 203 



If the earth, as it grew up, had been perfectly homogeneous, layer by 

 layer, as in the supposed fluidal earth, the whole effect of increased mass 

 might have been merely a uniform self -compression ; but since the tex- 

 ture and composition of the earth was very heterogeneous in a minute 

 sense and became regionally diversified in a large sense, by reason of the 

 external agencies that affected its growth, it was inevitable that distor- 

 tion should arise not only whenever, but wherever, self-compression took 

 place. It is thus a firm inference that diastrophism in some form perme- 

 ated, in a general sense, the whole of the self -compressing body, certainly 

 the whole of that part which was built of planetesimals. The concept of 

 merely surfical diastrophism, in response to general stresses in a body 

 having such a genesis, seems to be altogether untenable. Of course, there 

 were local distortions of all sorts, surficial as well as otherwise. 



The TOTAL Value of the Self-compression 



I have recently endeavored to determine the total amount of self-com- 

 pression which the earth suffered in the course of its history by means 

 of a comparison of its density, mass, and volume with the densities, 

 masses, and volumes of the planetary bodies nearest to it and most like 

 it in their conditions of genesis, namely, the next inner planet, Venus; 

 the next outer planet. Mars, and the earth's own satellite.* The method 

 consisted of building up the moon with moon-stuff, at the moon's density, 

 until its mass equaled that of the earth, when its dimensions and volume 

 were computed, after which it was theoretically shrunk to the density of 

 the earth and the amount of self-compression determined. Though not 

 the actual process of formation, this is believed to fairly represent the 

 degree of self-compression which would take place in a growing body 

 under terrestrial conditions between a stage represented by the moon and 

 a stage represented by the full-grown earth. So, in like manner. Mars 

 was built up of Mars-stuff at Martian density and the self-compression 

 between a Mars-stage and the mature earth-stage computed. So likcAvise 

 of Venus. The results were concordant in a very suggestive and satis- 

 factory way and were surprisingly large. Between the moon-stage- and 

 the earth-stage, the radial contraction was found to be 725 miles, the 

 equivalent of 4,555 miles contraction of the circumference. The shrink- 

 age between the Mars-stage and the earth-stage came out 618 miles radial 

 shortening; that between the Venus-stage and the earth-stage, 177 miles. 

 By a comparison of these results with one another, there came out the 

 further very important conclusion that the self-compression became pro- 



* Diastrophism and tlie formative processes. X. The order of magnitude of the shrink- 

 age of tlie earth deduced from Mars, \'enus, and tlie Moon. Jour, of Geol., vol. sxviii 

 (1920), pp. 1-17. 



