MOON AND RADIOACTIVITY — FORBES 213 



equilibrium. Suess ^ was one of the first to note the depressions in 

 wliich the maria occur. He observes that " The regular peripheral 

 surface fractures which surround the Mare Hunioruni at some dis- 

 tance from its margin, and other similar examples show, however, 

 that in some cases the circular subsidence extends beyond the surface 

 of the lava." The hypothesis herein adopted appears to be in good 

 accord with this observation. 



The moon having got rid of that fraction of its accumulated in- 

 ternal heat which brought about liquefaction of the substratum and 

 consequent convection currents, its crust is now a little too big for 

 its cooling and contracting interior. Some of its internal mass has 

 also been lost by extrusions of lava upon the surface. The fissures 

 originally formed in the extension of the crust can not now be 

 closed again by the efforts of the outer envelope to contract with the 

 shrinking substratum, for the fissures are filled with solidified lava. 

 Accordingly the weakest parts of the surface are subjected to buck- 

 ling and wrinkling as the more rigid parts of the crust are thrust 

 against them. As outlined in the previous paragraph, the maria are 

 particularly susceptible to further depression as the less fractured 

 portions of the crust exert a lateral pressure upon them. Due to 

 the same forces, the surfaces of the maria, while plastic are thrown 

 into the numerous anticlinal folds which they exhibit. A partic- 

 ularly fine example of this folding is visible in the Mare Serenitatis, 

 where a meridional anticlinal chain with syntaxis and linking of 

 arcs stretches from border to border. A meridional line of five small 

 vulcanoids on the face of the same mare points to a line of fracture 

 in the crust beneath the lava, which latter, subsequent to its extru- 

 sion, was pierced by minor eruptions along the line of fracture. 



The existence of mountain ranges in close association with the 

 maria, points to the fact that these too were caused by pressure in 

 the crust striving to accomodate itself to a contracting interior. 

 Where the rigid crust thrust itself most intensely upon the less rigid 

 maria, the edges of the latter were crumpled and thrown up into 

 the ranges which in many places border the maria. That the moun- 

 tain ranges were elevated after the craters is shown by the fact 

 that the latter are occasionally seen to be distorted or deformed by 

 the former features. 



The terrace effect within many of the craters, due to stages in the 

 sinking of the lava within them, may possibly be ascribed to lessen- 

 ing of internal pressure as an adjacent mare was poured out upon 

 the moon's face. Then each terrace would mark a pause in the for- 

 mation of a mare, and the drop in the lava would he produced by a 

 reduction in pressure due to renewed extrusions beneath the mare. 



•E. Suess, Das AntUti der Brdc. III. 2. chap. 28, p. 085. 

 2809&— ;n 15 



