MOON AND RADIOACTIVITY FORBES 211 



weakened from beneath as to be no lon<i^er strong; enough to resist the 

 internal pressure. Once hiva had pierced the crust, pressure within 

 would be lowered and rapid liquefaction and expansion would ensue. 

 This subcrustal liquefaction would be confined to a substratum 

 relatively thin when compared with the moon's radius. The lower 

 limits of this stratum would be determined by the interaction of heat 

 and the pressure of the superincumbent maj2jma and crust; the 

 upper limits by the conductivity of the surface layers. The volume 

 of the moon would be increased and lava would rise in the innumer- 

 able fractures caused in the crust. Where the fractures were rela- 

 tively narrow, lateral conduction in the cool upper layers of the 

 crust would congeal the lavas before they could issue upon the sur- 

 face. Where the fissures were wider, as we must suppose them to 

 have been beneath the maria, conduction was insufficient to solidify 

 the lavas, which thus were able to be extruded upon the surface. 

 Evidence of internal expansion is seen in the linear arrangement of 

 many groups of small vulcanoids, which mark lines of fracture in the 

 crust. A fine example of this is the line of craterlets over 200 miles 

 long between the vulcanoids Catherina and Abulfeda. 



Shaler* observes that ''those (fissures) which are in appearance 

 sufficiently conspicuous to be mapped lie mostly in tlie central part 

 of the visible surface, between the parallels of 30° north and south 

 of the moon's equator and within 30° east and 50° west of the cen- 

 tral meridian. They are thus remarkably rare in high latitudes 

 and apparently seldom near the east and west margins of the visible 

 part of the sphere. This apparent feature of distribution may be 

 due to the oblique view of these marginal fields." That this dis- 

 tribution is apparent rather than real as suggested by the previous 

 sentence is unlikely, for surely if obliquity of view tends to render 

 these features invisible, this effect should operate at equal distances 

 from the central meridian, whereas this is not the case. 



An explanation of this distribution of the most conspicuous frac- 

 tures may lie in the effects of tidal forces. The moon exhibits a 

 fossil tide which has been estimated to be too large for the orbit 

 it now occupies, the result of recession from the earth of the moon, 

 and the inability of the latter to adapt its form to its position with 

 reference to the earth. Liquefaction of the substratum would offer 

 an opportunity of rapid adjustment of the moon's figure. The rift- 

 ing of the proximal surface indicates that there the strains of adjust- 

 ment were greatest, and a like area of rifting probably exists on the 

 distal portion of the moon. Apparently the crust tended to frac- 

 ture more on the proximal and distal areas, where it became re- 



• N. S. Shal'T, A Coniparlsou of the PVatiirrB of tlic Earth and the Moon, SoiithBoniun 

 Contributions to Knowledge. Vol. XXXIV, 190.!. p. 45. 



