A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 23 



terrace-like structures on these walls such as would be produced by the succes- 

 sive stages of descent of the lava of the crater. Here again by the use of the 

 method of series we may intimately connect the vulcanoids of this group with 

 those of the two preceding groups. None of the students of this classification 

 whose writings are known to me has failed to observe that there exist examples 

 which may be classed as wall plains quite as well as ring plains. There is no 

 doubt that these ring plains have in general better defined, more volcano-like 

 cones than the wall plains, and that the contact phenomena of the lava of the 

 floor with the inner slope of the rampart are more characteristic of volcanic 

 action as we know it on the earth, yet these differences seem to me so to 

 graduate together in the two groups as to afford no basis for distinct classification. 



In the group of craters selenographers have placed so far the greater 

 number of the vulcanoid pits. They have included in them nearly all the 

 distinct pits from about fifteen to about three miles in diameter. So far as I 

 have found, they suggest no definite criteria for the members of this group, save 

 that they are widely distributed, occurring even on the walls of the large 

 structures, and that on this and other accounts they appear to be newer than 

 the wall plains or the ring plains. Inspection shows that there is no structural 

 difference between the vulcanoids of this and the preceding groups, their rela- 

 tively smaller size and apparent newness of formation affording no good basis 

 for instituting a category in which to place them. 



Following down in the order of size, the next accepted group is that of 

 crater-cones. The objects included in this category are all of small size. Elger 

 compares them to the parasitic cones of ^Etna, which seems to me not a happy 

 comparison, for their origin is in no wise related to the vEtna " parasites." As 

 the pits are generally less than a mile in diameter it is difficult to determine the 

 shape of their bottoms. My own observations agree with those of the selenog- 

 raphers, that these pits are usually in the form of inverted cones, terminating 

 downward obtusely, i. e., with no very distinct floors, and further that they are 

 occasionally found with rounded, saucer-shaped bottoms, as if there had been 

 lava in the cups, which had withdrawn with the cessation of activity into the 

 deeper part of the crust. There is enough of this obscure flooring to connect 

 by series the crater-cones with the craters, showing clearly that the difference 

 between the two is one of dimensions alone and does not indicate any essential 

 difference in the nature of the constructive actions. As regards the distribution 

 of the crater-cones and craterlets, it is to be noted that they in certain instances 

 appear to be associated with the light streaks ; of this feature we shall take 

 account hereafter. 



The smallest of the observable pits on the surface of the moon are termed 

 craterlets, or crater pits. These features are extremely numerous, the actual 

 number on the visible part of the sphere, which might under favorable conditions 

 be counted, amounting to many thousands. In the most characteristic specimens 

 of this group there is no distinct wall or cone surrounding the pit, the opening 

 often being abrupt, as if it were brought about by a mere subsidence of the area 



