A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 13 



they must owe their origin to other than volcanic action, for on the earth we find 

 volcanoes very generally disposed along lines which, in most if not all cases, 

 appear to be determined by faults. In many instances, however, the lunar vul- 

 canoids have a linear arrangement. 



The vulcanoids of larger size which are arranged in linear order are not 

 numerous. Among these may be cited the train extending from Herschel 

 through Ptolemffius, and Alphonsus to Arzachel ; that from Thibet to Stofler ; 

 that from Atlas to Franklin ; and that from Vendalinus to Casatus, near the 

 limb in the third quadrant. (See plates i and xxi.) In all these instances 

 there are four or more pits in fairly true alignment : in alignment and in number 

 they appear to exclude the supposition that their order is due to chance. Pass- 

 ing from the examples in which the greater vulcanoids are grouped in trains 

 and taking the pits of smaller size, we find the instances of such arrangement be- 

 coming more numerous as the structures are of smaller diameter. It is, however, 

 in but few of the pits over ten miles in diameter that there are more than three 

 or four so placed in relation to one another that they can be said to be linearly 

 arranged. 



When, in following down the series of vulcanoids as regards size, we come 

 to the pits less than a mile in diameter, those commonly termed craterlets, we 

 note that the linear order, hitherto exceptional, becomes so common that the 

 exceptions are rather to be found in the departures from it. The observations of 

 W. H. Pickering and others, as will be noted below, make it evident that there 

 is a causal relation between the smaller visible pits and the cracks that form on 

 the surface of the moon. There can be no question that there are thousands of 

 these smaller of the craterlets which are thus disposed in lines, some of the 

 series extending for hundreds of miles. (See plate xx.) 



It may be taken as evident, that in the time when the larger vulcanoids were 

 in process of formation the conditions of strain in the moon's crust were not such 

 as to determine that the points of outbreak should to any great extent be linearly 

 arranged and that when thus arranged they tended to follow the meridians, rather 

 than the parallels. In the later stages of the surface when the smaller openings 

 were made they obviously tended to a linear order, but the direction of the lines 

 was exceedingly varied, some of them being radially disposed with the greater 

 vulcanoids as centers, others along lines of weakness which lie in extremely 

 diverse positions. 



Reckoning great and small, there are some hundreds of these lines of pits, a 

 number sufficient to make it evident that they cannot be accounted for by chance. 

 It is evident that to explain this linear order of vulcanoids by the hypothesis we 

 are considering is difficult if not impossible, for that would require us to sup- 

 pose the bolides to have been thus arranged during their movements through 

 space. It is also to be noted that in very many instances there are pits within 

 the larger cavities so centrally placed that they cannot be explained by the chance 

 in-falling of bolides. Therefore, while the relation of lunar volcanoes to those of 

 the earth is a perplexing question, there seem on the face of the facts to be 



