212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



lieved of support by migration of subcrustal magmas in the direction 

 of like latitudes on the limb, than the latter areas where it became 

 subjected to a buoyant thrust. The present existence of an exces- 

 sive fossil tide, indicates either that during the moon's last phase 

 of volcanic activity its adjustment of form to its position was incom- 

 plete, or that the excess is a measure of the moon's recession from 

 the earth since the last phase of vulcanism. 



The maria occupy a third part of the moon's visible surface, and 

 most observers are agreed that these are outpourings of lava, but a 

 comparatively thin' layer upon the surface. Craters are relatively 

 rare in these areas, owing no doubt to the fact that all but the high- 

 est of them would be covered in the lava flows. That the formation 

 of the maria occurred after most of the major craters appeared is 

 inferred from the melting of crater walls by the maria, a phenome- 

 non visible in numerous places. Failure of the maria to coincide in 

 location with that area where fracture by tidal forces appears to 

 have been most intense, seems to indicate that their positions were 

 determined by another factor. Local fragmentation of the crust 

 and consequent extrusion of the maria may have resulted from a high 

 concentration of radioactive materials beneath those areas. Such 

 areas would be characterized by higher temperatures than elsewhere. 

 This would operate to thin the crust greatly, causing much fractur- 

 ing, and would also render the magma more fluid than elsewhere, 

 facilitating the flow of the extruded lavas. 



The maria occupy the loAver portions of the moon's surface. Bar- 

 rell '^ supposes this depression to have been accomplished by the 

 ejection upon the surface of magma from below the level of isostatic 

 compensation. This loading of the crust with material heavier than 

 any found above the level of isostatic compensation would cause a 

 slight subsidence of the crust, which depression would be enormously 

 increased by lateral compressive stresses. We have seen that the 

 maria probably occupy the most fractured portions of the crust, and 

 the addition of several thousand feet of lava, a relatively thin 

 covering, does nothing to reinforce the strength of those shattered 

 areas. As outlined in the following paragraph, lateral compressive 

 stresses are likely to occur at the close of a revolution, shortly after 

 the extrusion of the maria took place. These horizontal forces 

 would naturally find easiest expression in the shattered parts of the 

 crust covered by the maria. The maria are thus probably not in a 

 state of isostatic equilibrium, since by far the greatest part of their 

 subsidence must be ascribed to action of lateral compressive forces. 

 The supposition is that they are of recent formation, geologically 

 speaking, and are at present moving towards a condition of isostatic 



'' Annual Report Smithsonian Institution. 1928. pp. 295-29G. 



