A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 27 



those now existing are but the last of a long series which has been formed and 

 destroyed. Close attention to these features in the moments of good seeing, 

 which occasionally reward the observer, will reveal a series connecting such still 

 distinct though extensively demolished rings with other more numerous fragments 

 of circles which would not be interpretable save for the connecting links. 



It may here be said that the phenomena of dilapidation exhibited by the 

 relicts of ring walls in the fields of the maria differ essentially from what we find 

 on the outlying surface of the moon. In the last-named areas, the ruining of the 

 ancient ramparts has evidently been in large measure brought about by the 

 encroachment and possibly by some shearing pressure of later-formed vulcanoids, 

 which actions have broken down and shoved about the fragments of the once 

 complete circumvallations. In addition to these processes of burial and displace- 

 ment, there have apparently been at work some influences which have slowly 

 broken down the rings, so that they have lost the original steepness of their 

 profiles. In and on the borders of the maria we find evidence that the destruc- 

 tion was brought about by the immediate and swift assault of the originally fluid 

 material that now forms these plains of frozen lava. The rings are not deformed 

 but more or less broken down, in part breached, by the stroke of a tide of fluid 

 rock, as in the case of Doppelmeyer and Hippalus on the shores of the Mare 

 Humorum, or simply overflowed and melted down, as is the case with the great 

 unnamed ring north of Flamsteed, the more effaced ring between that structure 

 and Damoiseau, or the many other like instances in other maria. 



As we pass from the largest rings downward in the series towards the small- 

 est craters which have distinct floors, we note a progressive increase in the fresh- 

 ness and finish of these structures. The departures from the original form 

 become less frequent, the walls are less breached, and the slopes of the ramparts 

 steeper and more even. The interference of rings of like size becomes rare, so 

 that with those less than five miles in diameter it does not appear to occur. All 

 these facts point to the conclusion which finds expression in the writings of many 

 selenographers, that in general the larger the rings the greater their age. 



PHYSICAL HISTORY OF THE VULCANOIDS. 



Comparing the lunar vulcanoids with the terrestrial volcanoes and adding to 

 the considerations no more than a reasonable amount of conjecture, it seems 

 to me that we may interpret the phenomena as set forth below. In this explana- 

 tion care has been taken to introduce into the interpretation nothing in the way 

 of action that does not appear to be warranted by the processes of our own 

 sphere. 



It is, in the first place, evident that while the lunar vents indicate some 

 process of eruption it cannot be regarded as in its nature identical with that of 

 ordinary terrestrial volcanoes. These last-named craters are, while they remain 

 active, with rare and questionable exceptions, on sea-floors or near their shores. 

 What we observe in their action and their distribution leads us to believe that 



