A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 25 



thousands of distinctly observable lunar pits proof of the non-explosive nature of 

 their eruptions, but we have other evidence to the same effect in the lack of all 

 signs of ejected masses and of dust-showers, such as are the most striking phe- 

 nomena of terrestrial outbreaks. If we select any of the vulcanoids situated in a 

 region of much accidented topography, which evidently existed before the vent 

 was formed, and examine the surface about the opening, we readily note that it is 

 not masked as it would be in case it had been subjected to a succession of ash 

 showers such as come from a normal terrestrial volcano. In many instances I 

 have observed that there was no trace of such ash-covering up to the very foot 

 of the ring wall. Like evidence of a more affirmative nature is to be had in the 

 very numerous instances in which one vulcanoid cuts another. So far as I have 

 been able to note the details of these instances, the earlier existing crater, except 

 where its walls have been deformed by the encroachment of its neighbor, never 

 suffers from any distinct obliteration. Its ring wall craterlets, vents, terraces, 

 and other slighter features, which should be hidden or distinctly changed in 

 aspect by an accumulation of even a few score feet of ash remains, so far as can 

 be discerned, unaltered. When we remember that there has evidently been no 

 erosive action on the moon such as has normally washed away thousands of feet 

 in thickness of ash about yEtna and other large terrestrial volcanoes, we see how 

 clear is this evidence that the lunar vulcanoids have not been the seat of ordinary 

 volcanic explosions. 



The lack of considerable lava flows on the moon appears to be almost as 

 well established as the absence of ash ; in but a few instances have structures 

 which can possibly be classed as flows of really fluid matter proceeding from 

 craters been reasonably suspected, and these on inspection appear to be more 

 than doubtful. As will be noted below, the material in the craters appears not to 

 have had a high order of fluidity, so that it quickly consolidated on very steep 

 slopes according to my observations generally exceeding 20 of declivity as 

 soon as it passed out of the cup. None of the rills or other fractures appear to 

 have afforded passage to the interior fluid material ; they seem, indeed, to have 

 been formed long after the larger vulcanoids had ceased to be active. 



DISTRIBUTION OF VULCANOIDS. 



In considering the distribution of the lunar vulcanoids it is first to be noted 

 that, unlike those of the earth, they are scattered over the whole of its visible 

 surface. The fact that here and there all around the limb we may trace the 

 hither borders of great ringed plains fairly leads to the supposition that like 

 structures exist on the unseen portion of the sphere. Except that on the maria 

 there are no large vulcanoids formed since those great plains were produced, 

 probably none as much as ten % miles in diameter that postdate their fluid period, 

 there is little to be said concerning the distribution of these features on their 

 surfaces. There are, it is true, considerable areas of the lunar surface outside of 

 the maria where the only vulcanoids are the craterlets. With slight exceptions 



