A COMPARISON' OF THE FEATURES OF THE EARTH AM> THE MOOX. 71 



to be flat or slightly concave, as is usual in such structures, but quite convex, 

 indicates a change, for it must have been originally level as Schroter, as well as 

 Baer and Madler, so represents it. A study of this feature will convince any 

 competent observer of the moon, who has had experience with his own work and 

 that of his fellows, that the peculiarity might easily have been overlooked. So, 

 too, with the craterlets on the southwest side of Copernicus, which have not 

 found a place on Baer and Madler's map, and the continuation of the same 

 craters and a honeycombed appearance of the ground towards Eratosthenes, 

 which Schroter failed to notice. An inspection of the field with a better instru- 

 ment than those used by the above-mentioned selenographers will show that they 

 may well have searched it a score of times without having a chance to note these 

 rarely visible features. On the whole, the evidence for and against the sudden 

 appearance and disappearance of craters and craterlets, or of features in their 

 structures considered without reference to the probabilities of such changes 

 based on the moon's history, leaves us in a state of doubt as to the occurrence of 

 such accidents. I am inclined to think that the case of Linne is the strongest and 

 that the walls of that vulcanoid may have, in part at least, fallen into the original 

 cavity so as to leave only a small pit in its crater unfilled. 



If it be the case that the originally great ramparts of Linne have disappeared, 

 the event may be explained without having recourse to the theory of volcanic 

 action. Against the hypothesis of such action may be set the fact that, though 

 the moon is the subject of constant scrutiny, no trace of such explosive process has 

 been noted. Moreover, if there was volcanic action in the case of Linne, it appar- 

 ently must have consisted in an outpouring of very fluid lava, which formed the 

 extensive white patch that took the place of the previously existing rampart and 

 pit. In a word, the great wall must have been melted down into the flood. 

 When we consider the fact that none of the other vulcanoids shows a trace of any 

 such flows, that the evidence points to the conclusion that the lavas coming from 

 the interior of the sphere never freely stream forth but consolidate on slopes of 

 high declivity, we see how exceptional, and therefore improbable, is the occur- 

 rence of any such event. To the geologist it is inconceivable that in the late 

 stage of the moon's history such an effusion of extremely fluid rock could have 

 taken place. The explanation he would give may be set forth as follows : 



Assuming that the lunar crust as the seat of high and varied tensions of 

 contraction and expansion brought about its night and day, and that it abounds 

 in cavities due to the ejection of the large amount of material contained in the 

 ramparts of the vulcanoids, it is conceivable that from time to time ancient but 

 unstable adjustments may be suddenly disturbed. The state of the lunar surface 

 may in a way be compared' with that of a Prince Rupert drop, a globular bit of 

 glass greatly affected by stresses which any shock is likely to set in effective 

 action. Now, if on such a surface a meteorite should fall, say a body of some 

 tons in weight, no larger than many that have come upon the earth, the resulting 

 shock might lead to widespread movements that would cause the walls of a vulca- 

 noid to fall in. It is to be noted that there are many ill-defined pits on the moon 



