A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 15 



is, however, as before remarked, evident that no such falls as have formed the 

 hundreds of pits over ten miles in diameter which exist on the moon's surface 

 have occurred on the earth since the Cambrian age. 



The foregoing considerations justify us in rejecting the hypothesis of falling 

 bolides as a means of accounting for the so-called craters on the moon. There 

 are, however, certain other features of lunar surface which may be explicable by 

 the impact of large bodies falling from space. These we will now proceed to 

 consider. 



MARIA OR SEAS. 



A large part of the surface of the moon is occupied by the so-called maria or 

 seas. These are extensive irregular, indistinctly circular areas of relatively level 

 nature and of a perceptibly darker hue than the other more rugged fields. This 

 dark hue is shared by the floors of a number of the craters which lie near the 

 seas, as for instance by that of Plato, and more rarely by craters which lie remote 

 from their margins. Though vulcanoids exist on the maria of the moon they are 

 of relatively small size, none, in my opinion, which have clearly been formed 

 since the material of which the maria are composed came to its present level posi- 

 tion, exceeding ten or fifteen miles in diameter. So far as I have been able to 

 reckon, the proportion of these pits on the seas does not exceed one-fifth that we 

 find on the other part of the lunar surface. The average discernible inclination 

 of the surface of the maria is relatively so small they are more nearly true plains 

 than any equally extensive land areas on the earth. 



It is a noteworthy fact that the maria, though they occupy about one- 

 third of the visible part of the moon, i. e., including what is shown by the libra- 

 tions, rarely, if at all, lie on the margin, in positions enabling us to infer that they 

 are parts of like areas on the unseen portion of the lunar surface. On the western 

 limb of the sphere the so-called mare Australis is generally mapped as extending 

 around the margin, as it in fact does at certain stages of the libration, but under 

 the most favorable conditions the ordinary rough surface of the satellite appears 

 to me to be visible beyond this small mare, so that the statement as to none of 

 these seas crossing the limb apparently does not admit of exception. The ill- 

 named mare Humboldtianum is evidently a vulcanoid. It therefore appears 

 probable that if such maria exist on the unseen portion they are less extensive 

 than on the part of the orb which we see. 



The most interesting feature of the maria is found in their contact with the 

 higher, rougher surface areas which bound them. Whenever I have been able 

 to observe this contact in a sufficiently exact manner there appears to be good 

 evidence that the material of which their surfaces are formed flowed in against 

 or upon the rough ground as very liquid lava would do. In a general way this 

 fact had been often noted. It fills in the lower ground forming numerous bays. 

 In many instances, as, for example, in the case of Doppelmeyer, it distinctly ap- 

 pears to have melted down the side of the crater's wall next to it, and to have 

 filled the cavity to its own level. Whoever will inspect these lines of contact of 



