MOVEMENT OF SCKEE-MATEKIAL. - 237 



Let US consider a scree-talus one mile in horizontal length : let the 

 average height measured along a line of slope be 1000 feet, and the 

 average thickness of the surface-stones 6 inches. Let us suppose 

 that the stones on one half only of the scree-surface are in a posi- 

 tion to creep, and that, on the average, every stone on this half 

 creeps downward one-thousandth of an inch a day. Then, the total 

 amount of movement is equivalent to 1320 x 1000 cubic feet moving 

 through one thousandth of an inch, or 110 cubic feet through one 

 foot, every day. 



This is of course a mere approximation ; but it will serve to in- 

 dicate the order of magnitude of the movement contemplated. If 

 we had similar numerical estimates for the other moving agencies, 

 we should be able roughly to compare them in efficiency. The im- 

 portance of each cause varies, however, with the climate and so many 

 other conditions that it would be difficult to assert at any time and 

 of any screes that one cause is more efficient than another. In all 

 probability, creeping is very far from being the most important cause 

 of movement, yet it is possible to imagine conditions under which 

 it might in time be most effective, as in the stone-rivers of the 

 Falkland Islands, and at least one case where it may be almost the 

 only agent at work. 



I refer here to the conditions which probably obtain on the sur- 

 face of the moon, where, as is well known, there are no seas, no 

 appreciable trace of an atmosphere, and where the most diligent 

 telescopic search for many years has failed to detect the slightest 

 sign of present volcanic action. Deprived of the most potent agents 

 of geological change, there remain the effects which can be pro- 

 duced by sudden alterations of temperature. The change from the 

 intense heat of the lunar day to the cold of the lunar night or shade 

 being untempered by any intervening atmosphere, the strain that 

 results from the sudden cooling will be amply sufficient to break up 

 the surface of any known rock. In this way, screes must accumu- 

 late on the mountain-sides, and the surface blocks that are free to 

 move will creep gradually downwards, perhaps more rapidly than 

 they would do on the earth, owing to the absence of vegetation and 

 disintegrated rock upon the moon and the great range of tempera- 

 ture, long though its period be, to which its surface-rocks are ex- 

 posed- tinbalanced by volcanic action, the actual rate of degrada- 

 tion may even be greater than on the earth ; but, whether this be 

 the case or not, there can be little doubt that in great and sudden 

 alterations of temperature there exists a very important source of 

 change upon the surface of an otherwise dead world. 



Discussion. 



The Peesident expressed some surprise that the observations had 

 not been made before, and congratulated Mr. Davison on the neat- 

 ^ess of his demonstration. 



Prof. Bonnet agreed with the Author that changes of temperature 

 do cause many stones to change their level, and considered the in- 



