DIASTROPHISM AND THE FORMATIVE PROCESSES ii 



the basis of shrinkage. The parity-earths were supposed to shrink 

 until their mean densities were identical with that of the present 

 earth. The amount of this shrinkage is recorded in Table II in 

 terms (i) of the earth's radius in miles, (2) of the earth's circum- 

 ference in miles, and (3) of the earth's circumference in degrees, 

 *each of these being more convenient than the other in certain 

 specific uses. 



In building up the meteorite earth Farrington's mean specific 

 gravity of meteorites seen to fall was taken as the basis of compu- 

 tation.^ While the inclusion of only those meteorites that have 

 been seen to fall may not be strictly representative, it is Farrington's 

 view that this limitation gives the best definite figure that is 

 available. If the meteorites found but not seen to fall were included, 

 the specific gravity would quite certainly be too high, because 

 metallic meteorites are more likely to attract attention on account 

 of their unusual heaviness and the whitish color of the metal, and 

 because they are less liable to disintegration than the stony mete- 

 orites. Nevertheless, if all meteorites that have reached the ground 

 in observable masses were averaged, the mean specific gravity 

 would probably be greater than the figure given. On the other 

 hand, the surfaces of iron meteorites are notably pitted, due 

 probably to the exfohation of the stony parts, as these are less 

 tenacious than the metallic parts. A naked body sweeping about 

 the sun and likely to be in rotation is quite sure to be subjected to 

 those rapid changes of temperature which promote exfoliation. 

 The gravitative power of a meteorite is very small and hence these 

 exfohated chips would be likely to be thrown off into separate paths 

 and thereafter play the part of individual meteorites. It is thus 

 probable that the vast multitude of small meteorites that are 

 burned to dust in the upper atmosphere are much more largely 

 stony than metallic. This consideration probably offsets any 

 weight that ought to be given to the preponderance of metal 

 among the meteorites found some time after their fall. At any 

 rate the mean given by Farrington is the best available and is 

 doubtless near enough the true mean to give the right order of 

 magnitude to the results deduced from it. 



' 0. C. Farrington, Jour, of Geo!., V (1897), pp. 126-30. 



