A COMPARISON OF THK FEATURES OF THE EARTH AND THE MOON. 31 



of a plug of frozen lava which fills the cup. If we suppose a ring widening by 

 the process of melting and rebuilding its walls, we may conceive that the fluid 

 is likely to extend at points beneath the ramparts, so that when, after a period 

 of repose, in which the lava was frozen and had shrunk, activity was resumed, 

 the easiest way upward for the vapors would be by passages leading vertically 

 through the wall. 



The curious fact may here be noted, that in no observed instance is there 

 distinct evidence of any lava flow which has broken under and through the ram- 

 part or cone surrounding a vulcanoid. When we consider that practically all the 

 lava streams from terrestrial volcanoes break out through the base of their cinder 

 cones, this condition of affairs on the moon demands an explanation. This may, 

 like many other of the lunar events, be explained by the fact that the weight of 

 the fluid, which is the impelling agent of its flowing, is only one-sixth that of 

 terrestrial lavas, while the cohesion of the rocks may be, and most likely is, quite 

 as great as on the earth ; certainly these cones, which apparently are far more firmly 

 built than the ash heaps of volcanoes, must have resisted the relatively slight 

 hydrostatic pressure of the lavas they enclose far better than the like structures 

 of the earth. 



We may here turn aside for a moment to consider the hypothesis that the evi- 

 dent and often probably repeated up-and-down movement of the lava in the vulca- 

 noids was due to tidal action effected by the earth. While it cannot be doubted that 

 the effect of the earth's attraction, at present six times as great on the moon as is 

 that exercised by that body on our sphere, and may of old have been yet greater, 

 would be competent to lift any internal united mass of fluid to a considerable 

 height, there are reasons why it cannot well have served to pump the lava up to the 

 elevations it attained in the lunar craters. To be operative, we have to suppose 

 that the terrestrial attraction took effect in a central mass of igneous fluid, the sur- 

 rounding crust being essentially rigid, not flexing to any great extent with the 

 pull, which seems to be an unwarranted assumption. Under these conditions the 

 lava would mount and descend in each lunar day, which, before the moon ceased 

 to have a diurnal rotation, may have been of almost any length less than what 

 exists at present which we have a fancy to reckon. It is, however, to be observed 

 that the lavas of the vulcanoids, from time to time, froze at exceedingly varied 

 levels, there being a range of several thousand feet in altitude in craters which 

 are near to one another. These stations of repose, long enough to permit the 

 freezing, are not to be explained on the hypothesis of incessant tidal pumping ; 

 nor have I been able to account for the facts by any warrantable subsidiary 

 hypothesis. Moreover, the smaller vulcanoids, the craterlets, which are evidently 

 in the same series as the greater, having little or no lava in their bases, cannot be 

 thus explained. Furthermore, the central cones of many of the larger vulcanoids, 

 the formation of which was evidently in some way connected with the actions 

 which built the whole structures, apparently cannot be brought under this 

 explanation. 



The most reasonable view as to the interior condition of the moon when its 



