562 PROCEEDINGS OF THE ACADEMY OF [Sept., 



be impact craters, are vastly larger than our Arizona crater, and 

 one of them is even 150 miles in diameter. When one who is familiar 

 with the Arizona crater examines the Imiar craters through a good tele- 

 scope they are at once seen to show the main features of the former. 

 The relation of width to depth is the same. Most of the ejected 

 material lies close around the lunar craters and forms the so-called 

 rim, as in the case of the Arizona crater. There are spurts or tongues 

 of ejected material in the Arizona crater and presumably in the lunar 

 craters. Even the peculiar conical central hill or mountain which is 

 observed in most of them and which I confidently assert cannot 

 "be explained on any theory of volcanic action, has its counter- 

 part in our own Silica Hill at Meteor Crater (see Plates XXI 

 and XXII). It probably exists in all of the lunar craters, but 

 in the very small ones it is not easily discernible on account of 

 the smallness of the crater and because, as in our crater, the effect 

 has been somewhat masked. This hill in the Arizona crater is now 

 somewhat masked by the overlying lacustrine sediments and by 

 fine material deposited by the action of wind over it. For a long 

 time its origin puzzled us greatly. It now seems to have been a neces- 

 sary feature of the impact. These central conical hills or veritable 

 mountains in the larger lunar craters Avould seem to be due to the 

 same physical law which we see in operation when we drop a stone 

 into water or soft mud, with Avhich solid rock can be compared 

 if the projectile strikes it at sufficient speed. A raindrop falling 

 on still water produces for a moment the same small conical-shape 

 in the centre of the cavity caused by the impact. In the case of 

 water, of course, it soon mingles with the surrounding water; in the 

 case of rock fragments or rather stiff mud it remains. In this connec- 

 tion one should read .4 Study of Splashes, liy Professor W. A. Worth- 

 ington, of Devonport, England, in which the author has introduced 

 some quite wonderful photographs and arrived at certain conclusions 

 with regard to the l^ehavior and flow of solid substances under great 

 pressure, suddenly applied, being analogous to the motion set up 

 in liquids or viscous material upon impact. These conclusions 

 seem to be fully warranted and also seem to go far toward ex- 

 plaining the presence of the conical-shaped hills in nearly all of 

 the lunar craters. Anyone who will make a careful study of our 

 Arizona crater and will then read Worthington's book, studying the 

 diagrams he has made, and will then turn his attention to the lunar 

 craters, cannot escape the conviction that the lunar craters are im- 

 pact craters. Why the moon should have been so abundantly bom- 



