MOON AND RADIOACTIVITY — FORBES 215 



the surface upon which the rrmria were extruded; but these fissures 

 were at right angles to the compressive forces. These forces caused 

 the magma in the deeply covered fissures to be squeezed upwards, 

 ridging the surface of the maria. This will explain the absence of 

 accompanj'ing synclines, which one would expect to find if the 

 ridges had been formed by a simple wrinkling of the maria due to 

 lateral pressure. 



Barrell ^" advances strong evidence in support of the assumption 

 that the lunar craters were not formed all in one phase of vul- 

 canicity. In the younger craters there is a sharpness of definition 

 and a steepness of slope which is absent in the older craters. Craters 

 of the former type are frequently superimposed upon the latter. 

 The ancient craters give evidence of previous revolutions in the 

 same way as do the stumps of denuded orogenic belts upon the 

 earth. 



The freshness of many of the features on the lunar surface and the 

 persistence of color gradations, induces the conclusion that a rev- 

 olution has occurred recently, geologically speaking. The moon is 

 now in a quiescent condition accumulating heat in the substratum, 

 and therefore does not radiato any perceptible heat. As time ad- 

 vances, the lunar features will inevitably lose their sharpness of 

 detail and gradation of color, until a further period of vulcanism 

 restores the surface to the condition which it now exhibits. During 

 this future liquefaction of the substratum, the present excess ma- 

 terial in the fossil tidal protuberances will tend to withdraw in the 

 direction of low latitudes on the limb, with a renewed fracturing 

 of the crust on the proximal and distal surfaces. Since the moon 

 will by then have retreated further from the earth, its fossil pro- 

 tuberances will be more disproportionately large than they are now. 



If we adopt the not entirely unreasonable assumption made earlier 

 in this article, that the moon during part of its early history pos- 

 sessed an atmosphere similar to the present terrestrial atmosphere, 

 then the fact that no traces whatsoever of water action have ever 

 been recognized upon its surface requires explanation. If we assume 

 the original surface features to have been of nonvolcanic forma- 

 tion, then the atmosphere may have become diffused into space 

 before the eflFects of vulcanism were impressed upon the ancient sur- 

 face. This vulcanism could be accounted competent to destroy any 

 traces of water action which might previousl}' have existed. The 

 conception of a revolution has had to be introduced to validate the 

 hypothesis. 



If, on the other hand, we assume that the original features of 

 the moon were volcanic in formation, then, as previously set forth, 



"Op. clt., pp. 304-.'i05. 



