598 GEOLOGY. 



tion of a class of intrusions which develop explosive phenomena by closc^ 

 approach to the surface, without actually reaching it. I 



Lunar craters. — There are grounds for thinking that the remarkable 

 craters of the moon, assuming that they are truly volcanic,^ may belong 

 to this class, for they are very similar to the Coon Butte pit. The capac- 

 ities of the lunar craters, so far as they can be estimated, seem to equal, 

 if they do not in many cases exceed, the volume of matter in their rims. 

 They do not appear usually to be great cones of accumulated material 

 with relatively small craters, like the typical products of terrestrial vol- 

 canoes. Besides, there are no clear evidences of lava-streams. The 

 radiating tracts once interpreted as such have been shown by increased 

 telescopic power and the resources of photography to be at least some- 

 thing other than lava-streams. They are vaguely defined tracts which 

 run over heights and depths indifferently, and are plausibly interpreted 

 as lines of debris projected to extraordinary distances because of the 

 absence of a lunar atmosphere, and because of the low force of the moon's 

 gravity. Since the moon now has no appreciable atmosphere or sur- 

 face-waters, and since it is doubtful whether it ever possessed either on 

 account of its probable inability to hold atmospheric gases or the vapor 

 of water in the form of an envelope about it, owing to its low gravity, 

 there is reason to suppose that the external matter of the moon derived 

 from the explosions of the multitude of lunar volcanoes would remain 

 in a loose, incoherent condition, from the absence of dissolving and 

 cementing agencies. It is reasonable to suppose that lava-tongues 

 arising from the deeper interior would have a higher specific gravity, 

 even in their heated condition, than this porous covering of the moon, 

 and that therefore they would almost universally become intrusions 

 rather than extrusions, or at most they would not rise beyond the 

 bottom of the craters they had produced by explosion. This seems to 

 furnish at least a plausible explanation of the prevaihng differences 

 between the large lunar craters encircled by mere rims and the much 

 smaller terrestrial craters seated in relatively large cones. 



">■ Gilbert, after a careful study of the moon's topography, has suggested that tht- 

 lunar pits may be indentations produced by infalling meteorites or planetoids, and 

 has shown by experiment that pits of a similar type, with similar central cones, can be 

 produced by impact. The Moon's Face: A Study of the Origin of its Features. 

 Presidential address, Phil. Soe. of Washington, 1892, Bull. Vol. XII, pp. 241-292. 



