THE MOON 339 



crust was thin. After this period was over, a second era of vol- 

 canic activity began. The craters produced, however, were much 

 smaller than those of the first period, and indeed are comparable 

 in size to some of our own larger terrestrial craters. They are 

 found chiefly upon the maria and smoother crater floors, and 

 may be recognized by their dark color. 



That there never was very much water on the moon's sur- 

 face is rendered certain by the lack of extensive areas of erosion. 

 Nothing is seen at all comparable in extent to what an outside ob- 

 server would notice in inspecting the earth under similar cir- 

 cumstances. It is probable that all or nearly all of the large 

 craters were originally furnished with central peaks, but in most 

 cases these were destroyed when the floors were liquefied. Of 

 the peaks which are left, doubtless all at one time, as in the case 

 of our terrestrial volcanoes, gave out large quantities of steam and 

 volcanic gases. On account of the rare lunar atmosphere, due 

 to the small force of gravity, this moisture would have been im- 

 mediately precipitated as rain or snow, so that the erosion would 

 have been confined to, and concentrated about the vents. 



In the lunar craters the only evidences of erosion are in 

 fact found on these central peaks, but so marked is the effect in 

 these places, that only a minority of the smooth cones of large 

 size are left, while in the majority of cases we find that they 

 have been partially cut, and sometimes almost completely worn 

 away. * * * 



It would seem as if there could be but little doubt that the 

 brightest regions shown in these drawings owe their brilliancy 

 to snow. In order to withstand the evaporation due to a tropical 

 sun, unprotected by any appreciable atmosphere, in latitude 11, 

 this snow must be rather deep, even if it is being constantly re- 

 newed from the Moon's interior. It must be remembered that 

 none of the bright lines shown can be less, allowing for irradia- 

 tion, than 50 to 100 yards in width, and none of the circles much 

 less than half a mile. In a rough and jagged region there would 

 be many small steep areas where snow would not lie, but which 

 would become visible to us only, on account of their small size, 

 by a diminution of the general brilliancy of the whole area. We 

 might naturally expect therefore that some snowy areas would 

 appear to be brighter than others. * * * 



A very casual examination of the Moon will show that un- 

 der high illumination nearly half of the craters exhibit white 

 patches on portions of their interior slopes. Very little attention 

 has ever been paid to them by selenographers. They are some- 

 times visible as soon as the Sun rises upon them, but more often 

 they gradually develop under the action of the solar rays. They 

 lie on the slopes most directly exposed to the Sun, and as the 



