DIASTROPHISM AND THE FORMATIVE PROCESSES 15 

 THE STAGE OF MAXIMUM SHRINKAGE 



If the four bodies under comparison are to be regarded as 

 representing stages of growth, it is a matter of much added interest 

 to deduce from the comparison the stage of growth at which the 

 greatest shrinkage took place. If the bodies were entirely compact 

 at the outset, as they would be if fluid, or pliantly viscous, shrinkage 

 from gravitative pressure might be expected to decline with every 

 stage of compression reached, because resistance to compression 

 usually increases rapidly as compression proceeds; but if the 

 material of growth were minutely fragmental at the outset and the 

 particles rigid and elastic, other factors of importance would come 

 in. At first the porosity would be great. Until the porosity was 

 exhausted the shrinkage would depend largely on the rigidity and 

 the elastic qualities of the constituent particles. Later, the 

 possibilities of chemical, crystalline, and physical readjustments in 

 the interest of density would come into service. Another factor 

 is the presence or absence of effective wash, solution, and redepo- 

 sition. These are dependent on the presence or absence of an 

 effective hydrosphere. The moon has neither appreciable atmos- 

 phere nor hydrosphere and if originally built up of minute rigid 

 particles it would retain a deep porous zone of relatively low 

 specific gravity. This would notably affect its mean density. 

 Besides, there is evidence of much explosive eruption and the 

 pyroclastic products arising from this would, in the absence of wash, 

 solution, and redeposition, remain highly porous. • The Mare once 

 regarded as seas and later as lava plains may perhaps really be 

 tracts of volcanic ash. Lines of projected debris crisscrossing 

 Mare Imbrium are well shown in a recent photograph by the 100- 

 inch reflector of Mount Wilson. Mars is on the ragged edge of 

 doubt; it may perhaps have enough water on its surface to wash 

 fine material from the exterior into the interior and to dissolve 

 more or less of the surface material and deposit it in the pores below, 

 or, on the other hand, the water may be so scant as to have Httle 

 effect in cementing and solidifying the outer zone of the planet. 

 But in the case of Venus, inwash and cementation are probably 

 efficient, while they are known to be on the earth. All these factors 

 seem to have played important parts in the results. 



