A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 35 



and in the unnamed district between the Lacus Somniorum and the Mare Crisium. 

 As it is the habit of the ridges to be rather straight, the occurrence of curved 

 fragments, varying from those of a few degrees of arc to half circular, appears to 

 warrant the hypothesis that antecedently existing vulcanoids have been broken 

 up in this peculiar constructive work. 



In some instances vulcanoids which were evidently once fairly perfect, as 

 such structures necessarily are at the time of their formation, have been appar- 

 ently invaded by the mountain ridges. This is the case in Marco Polo, just 

 above mentioned. Here an originally normal ring plain has been broken into 

 on its northern versant, and thereby so deformed that its original nature is not 

 readily perceived on casual observation. The great walled plain of Hipparchus 

 appears to have been in large measure destroyed by the development of mountain 

 ridges, which traverse its walls and in part the enclosed plain. Many other 

 instances could be cited to show that these mountain-building actions, whatever 

 their nature may be, have been very effective in deforming if not in destroying 

 the vulcanoids of large area. Even the generally well-preserved Plato appears to 

 me to exhibit in its wall evident traces of dislocation arising from the disturbance 

 of the moderately accidented region about it. 



There is no evidence sufficient to determine the stage when the building of 

 lunar mountains ceased. There is, however, reason to su.spect that they were not 

 formed after the maria came into existence. There are, it is true, a number of 

 groups of such structures which lie within the boundaries of the seas, but there is 

 some reason to believe that these are the survivals from an antecedent time, being 

 parts of systems which were not entirely buried by these widespread lava fields, 

 though they show to my eye distinct evidence of having been effected by the 

 inundations of liquid rock. If this judgment as to the history of the intramarian 

 ranges be accepted, then we may safely conclude that the mountain-building 

 period was passed before the seas were formed. There is some reason to suppose 

 that this stage of the lunar development did not extend down to the time when 

 the smaller vulcanoids, at least those which lie outside of the ring plains, were 

 produced. In no instance have I observed any of the mountainous folds break- 

 ing in upon craters less than ten miles in diameter, though my observations are 

 not sufficient to completely exclude such occurrences. In many instances, how- 

 ever, very well-shaped craters of several miles in diameter occur in mountain-built 

 areas. They often are so well preserved that we have to exclude the supposition 

 that they were formed before the ridges were developed. 



The second group of prominences which may be termed mountains has for 

 its type the isolated masses which often occur in the central parts of lava floors 

 of the greater vulcanoids, and more rarely in excentric positions on those floors. 

 These reliefs were evidently produced by some action connected with the forma- 

 tion of small craters which they appear to replace. Such craters on the floors 

 of the vulcanoids are, as is well known, extremely common ; in many instances 

 there are more than a dozen within the ring, and in the Stadius Schmidt says he 

 counted fifty, and forty-one have been delineated. Commonly there is either a 



