THOMSON. — THE FALL OF A METEORITE. 731 



spread more generally and uniformly around the crater instead of in 

 streaks. The appearances suggest a sudden splash. The central moun- 

 tain or hill, so common a feature of the larger lunar craters, shows no 

 volcanic vent. Its existence is then more compatible with the idea of 

 a reaction, or " kick back," an inrush similar to that which occurs on 

 dropping a stone into a pool of water, except that in the case of the 

 lunar crater it is composed of solid fragments or more or less pulver- 

 ized rock. This central hill is absent in the smaller craters, as in 

 those comparable in size with the Arizona Crater. The most ancient 

 craters on the moon have had time to become softened in outline and 

 in a measure obliterated by cosmic dust. This may be the reason why 

 so few show the system of radiating streaks so characteristic of Tycho 

 and Copernicus. A thin lunar atmosphere, afterward lost, may have 

 prevented their formation. The fact that the craters of Tycho and 

 Copernicus are so clean cut, so sharply defined, so deep, free from par- 

 asitic craters or subsequent impacts, and still surrounded by the exten- 

 sive, unobliterated system of streaks, would in this view, make them 

 the result of the very latest or more recent large impacts which the 

 moon has sustained. 



The hypothesis of meteoric origin for the lunar craters has been com- 

 batted on the ground of the absence of tangential or grazing impacts. 

 There may be some reason for this absence. If most of the effect was 

 due to the fall of bodies moving in a general path similar to that of the 

 moon, we could understand the case as one in which the gravitation of 

 the moon itself tended to change the path from a horizontal direction 

 and so prevent grazing impact. There are, however, some grooves or 

 furrows on the moon's surface the most perfect example of which is the 

 Great Valley of the Lunar Alps which looks very much as if made by a 

 projectile at last arrested and in part deposited on the plain beyond. 

 Near this also is the formation called the Straight Range which is 

 remarkably suggestive of a glancing blow ; as if a large body had failed 

 to embed itself on the first impact and had tumbled along, ruffling the 

 surface and shedding debris until its velocity was at last checked. 



If it be correct to regard the moon as having been separated from the 

 earth early in the history of our planetary system, there must have been 

 a large amount of scattering at the time of separation. The earth would 

 have retained the gases and water vapor almost wholly so that the moon 

 would have gone off without an atmosphere. The moon and the earth 

 also would gather up most of the material so scattered and the moon 

 might then have accumulated its scars, or at least the earlier ones. The 

 fall of any large mass would of course obliterate all smaller indentations 

 made before its occurrence not only at the place of impact but for some 



