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



in which it lies. Yet here, too, there is a gradation, for in sundry instances there 

 is trace of a ring wall as if some material had been extruded. In many instances 

 these pits are not circular, but with irregular outlines, which further suggest that 

 in certain cases there was no explosive discharge, but an in-falling of the covering 

 of a pre-existing cavity. It is further to be noted that these craterlets often, per- 

 haps oftenest, lie upon ridges, either the walls of the larger vulcanoids or the 

 numerous elongate elevations which occur in great numbers on various parts of 

 the surface and appear not to be connected with any large vents. In general 

 it may be said that the craterlets are the smallest observable members of the 

 series which has for its largest term the ring plains, and that they are among 

 the newer features of the lunar topography. 



Looking upon the variety of form of the vulcanoids of the moon in the light 

 of our knowledge concerning the shape of terrestrial volcanoes, it may be said 

 that the range in form is not very much greater in the case of the satellite than 

 in that of the planet. Between the great caldera craters, such as those of the 

 Sandwich Islands or the Bolsena group of Italy on the one hand, and the smaller 

 cones on the flanks of .^tna on the other, we have a range in width of cup less 

 considerable but approaching what is found on the moon ; or, comparing the 

 nearly coneless craters of the Eifel, the products of a single eruption, with 

 the peaks of the Teneriffe type or those of the Andes, we note a difference in the 

 ratio of the enveloping cone to the interior which is also comparable to that 

 exhibited by the lunar vulcanoids. It is evident that the series of lunar craters 

 has much ampler range in diameter than those of the earth, but the correspond- 

 ences are sufficiently evident to justify us in including all such features of our 

 satellite in one group, assuming that the conditions of their formation were prob- 

 ably as near alike as in the several varieties of terrestrial volcanoes. An inspec- 

 tion of the lunar vulcanoids shows us that the most important features which 

 separate them from those of the earth are to be found in the amount and nature 

 of their extrusions ; the order, or lack of it, in their positions on the surface ; and 

 the influences which have served to deform or to destroy their features. These 

 peculiarities will be considered below. 



The presence of a level surface of frozen lava in all of the lunar vulcanoids 

 save perhaps the very smallest is, as compared with the volcanoes of the earth, 

 their most conspicuous feature. This clearly indicates the relatively languid 

 nature of the eruptions from those craters. There are, it is true, a number of 

 terrestrial volcanoes where such a floor exists, but in all cases the facts justify us 

 in supposing that the last eruptive action was of the milder type, as in the case 

 of Kilauea in the Sandwich Islands. Eruptions of even slight intensity meas- 

 ured by terrestrial standards result in blowing out all of the fluid rock. Thus we 

 are justified in regarding the level interiors of these vulcanoids as evidence that 

 the normal lunar crater did not discharge explosively in true volcanic fashion. If 

 such violent discharges took place at any stage of the history of our satellite they 

 appear to be unrecorded in its existing features. 



Not only is the presence of lava shaped on a floor in all the hundreds if not 



