104 ROUGH WAYS MADE SMOOTH. 



mysterious ray-systems than that they were caused by what 

 Humboldt (speaking of the earth) calls the reaction of the 

 interior on the crust Nasmyth has admirably indicated their 

 appearance, or rather their radiating form, by filling a glob- 

 ular glass shell with water, hermetically closed, and then 

 freezing the water. The expansion of the water bursts the 

 glass shell, and the lines of fracture are found to extend in 

 a series of rays from the part of the shell which first gave 

 way. But this experiment of itself does not explain the 

 mystery of the lunar rays. Accepting the theory that the 

 moon's crust yielded in some such way, we have still to 

 explain how the rifts which were thus formed came to be 

 covered over with matter lying nearly at the same level as 

 the surrounding surface. It appears to me that the only 

 available way of explaining this is somewhat as follows. 

 First, from the way in which the streaks are covered like the 

 surrounding region with craters, we may conclude that the 

 streaks are older than any except the largest craters ; from 

 the great extension of many of them, we may safely infer 

 that the lunar crust possessed a large measure of plasticity 

 when they were formed (for otherwise it would have yielded 

 over a smaller area). It was, therefore, probably still hot 

 during the era (which may have lasted millions of years) to 

 which the formation of the rifts belonged : accordingly the- 

 lava which flowed out through the rifts remained liquid for a 

 considerable time, and was thus able to spread widely on 

 either side of the rift, forming a broad band of lava-covered 

 surface, instead of a steep and narrow dyke. This seems not 

 only to account for the most striking peculiarity of the bands, 

 but to accord well with all that is known about them, and 

 even to suggest explanations of some other lunar features 

 which had appeared perplexing. I understand that in 

 certain regions of North America there are lava-covered rifts 

 large enough to form geographical features, and, therefore, 

 fairly comparable with the lunar radiating streaks. But as 

 yet American geologists have not presented in an accessible 

 form a description of the peculiar features of the American 



