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



walled plains, those who use this classification include the greater pits with the 

 ring of high land about them. Elger selects Ptolemaeus as the type of this 

 group. He states that it is the distinguishing characteristic of this group that 

 there is " no great difference in level between the outside and the inside of the 

 walled plain " ; he proceeds to cite notable exceptions to the rule, accepting 

 Schmidt's term of transitional forms for them. These many exceptions range 

 from Gassendi, where the interior plain lies at about two thousand feet above 

 the floor of the Mare Humorum, which three-fourths surrounds it, to Clavius, 

 where the interior is some three thousand feet below the general level of the area 

 in which it lies ; such variations are so numerous that they include practically all 

 the differences in the altitude of the enclosed plain which we find in any of the 

 groups. Nor are the other criteria of this category more characteristic. The 

 irregularities in the walls, the clefts, breaches, and greater breaks, are, in propor- 

 tion to the length of the encircling ridges, hardly more frequent than in the 

 mountain rings or ringed plains. So, too, with the minor craters, cones, 

 and ridges on the floors and rims ; they are abundant, as inspection proves 

 roughly, in proportion to the area and the age of the structure. A careful ex- 

 amination of this group of walled plains will satisfy the observer that they are 

 essentially like the mountain rings except for certain accidents which have be- 

 fallen the members of the last-named group. 



Nearly all the so-called mountain rings, all, indeed, that I have been able 

 to group in this category, lie in the maria. They appear, as has been considered 

 by several selenologists, notably by Elger, to be the more or less ruined rem- 

 nants of what were originally to be classed as walled plains. From their posi- 

 tion in the maria and even more from their topographic features, they are fairly 

 to be regarded as akin to the first-named group in origin and general history, 

 save that at the time when the maria were in igneous fusion their rings were in 

 part melted down and it may be in part breached by the tides of lava which 

 surged against them. In some instances these mountain rings appear to have 

 been suffused by the lava when it stood at its highest level, and afterwards bared 

 as the surface of the fluid was lowered. The maria of the second and third 

 quadrant particularly abound in these structures, in every stage of assault and 

 demolition, from those which stood so high above the flood of lava that their 

 exterior slopes show only slight signs of attack, to the intermediate stage of the 

 broken ring immediately north of Flamsteed, and thence to sundry unnamed and 

 scarcely recognizable fragments of rings in other fields of the maria. There 

 seems, indeed, hardly any room for doubt that to establish this group we 

 shall have to accept the principle that the state of obliteration of lunar forma- 

 tions affords fit basis for their classification. It appears to me that for my 

 purpose this group must be rejected. 



In the group of ring plains selenographers have grouped all the strongly 

 walled vulcanoid pits of the lunar surface ; they find the criteria for separating 

 them from the walled plains in the more continuous nature of their ramparts 

 and the steep declivity of their inner walls. They note also that there are often 



