64 A COMPx\RISON OF THE FEATURES OF THE EARTH AND THE MOON. 



Many of these pits which may be seen in advantag-eous conditions are not more 

 than four or five hundred feet in diameter, and seem to have the general shape 

 that vi'ould probably be given them by the sudden effectively explosive develop- 

 ment of gases which we have seen reason to suppose would be brought about 

 by the penetration of large materials into the crust. Yet as there is no indi- 

 cation of a peculiar coloration of the fields about those pits, such as would be 

 produced by the precipitation of the condensed vapors, this interpretation must 

 be regarded as unverified, though it remains possible. Taking into account the 

 fact that the best instances of the honeycomb type of pits occur in tolerably clear 

 relation with the larger vulcanoids, it seems most likely that this group of depres- 

 sions owes its origin to the escape of indigenous vapors from the depths of the 

 lunar sphere. 



The question as to the possibility of any of the distinct vulcanoids owing 

 their formation to the impact of large meteoric bodies is elsewhere discussed. 

 It is therefore only necessary here to note that, as the size of the in-falling 

 body increased, the heat evolved would be augmented, so that a mass a few hun- 

 dred feet in diameter would inevitably bring about such a general melting of 

 the crust where it fell that a cavity would not be formed, but in its place a 

 level blotch caused by the frozen lava, substantially what we find in the maria. 

 There are, indeed, sundry patches on the lunar surface which may have this 

 origin, but so far I have not been able to find any criteria sufficient to warrant 

 this interpretation of them. 



The eminent probability that the fall of meteoric bodies on the lunar surface 

 should lead to the temporary production of a high temperature, suggests that it 

 might be possible by photographic if not by eye observations to detect these col- 

 lisions, if they occur with anything like the frequency per unit of area with which 

 they come to the earth. It is possible, though not likely, that these observations 

 might be practicable on the illuminated surface of the satellite, for the reason, 

 elsewhere noted, that as a whole it is more nearly black than white, and even a 

 small meteor would at its contact with the surface be likely to produce a flash 

 sufficiently brilliant to make an impression on a sensitive plate. On the dark 

 part of the sphere or even in a lunar eclipse it would probably be easier to make 

 the photographic observation. It is, however, to be noted that, as meteors enter 

 the crust at high speed and there is no atmosphere to give the train of light such 

 as is exhibited by those of small bulk which fall upon the earth, the flash might 

 be of very brief duration — so brief, indeed, that it might escape the eye and the 

 camera alike. 



It may well be observed that, supposing the moon's surface to have received 

 extensive contributions of meteoric matter, we might thereby possibly explain the 

 apparent degradation of some of its older features. On the supposition that the 

 in-falling bodies penetrated deeply and were converted into the gaseous state so 

 that they produced explosions, we would have an agency competent to break 

 down reliefs in the manner in which many of the ancient features seem to have 

 been mined. Yet when we note the exceeding sharpness of outline retained by 



