68 A COMPARISON OF THE FEATURES OF THE EARTH AN D THE MOON. 



temperature of the lunar day is that fragments lying on steep slopes would slowly 

 move down the declivities. Such detached blocks would, where they expanded and 

 in proportion to the efficiency of the gravitative impulse, press more vigorously 

 against the obstructions below them than on those above ; they would thus gain 

 a chance to creep farther downward when they were again expanded. This 

 process would somewhat resemble what takes place where a talus slope is knit 

 together by a sheet of snow ice, when we may note a creeping of the united mass 

 due to the changes of temperature it undergoes. I have frequently observed 

 taluses where this process has extruded the deposit, as in the manner of a glacier, 

 far beyond the limits to which masses falling from the cliffs whence they came 

 ever attain. This process is yet more nearly alike to that which takes place in 

 the lead covering of roofs, where the metal has been observed slowly to work 

 down the slopes on which it lies in a movement evidently due to alternating 

 expansions and contractions. 



At first sight it may seem that the relatively small value of gravity on the 

 surface of the moon would limit the movement of fragments due to expansion 

 and contraction so that the angle of repose in the taluses they formed would be 

 very high ; but on consideration it appears to me that this angle may be even 

 lower than in terrestrial conditions, for the lessened weight of a given volume of 

 rock would greatly diminish the amount of the friction, and the value of the adhe- 

 sions which tended to resist its movements would, owing to the absence of water 

 and chemical decay, be so slight that I see no reason why, given time enough, the 

 talus material should not be brought to a nearly level attitude. The ^coefficient 

 of expansion is likely to be the same in lunar materials as in the igneous rocks on 

 the earth, while the resistances to such motion, both in the horizontal flakes of 

 great width and in the detritus on steep slopes, would be but one-sixth what it 

 is in our sphere. Therefore we may reckon on this agent of change being of 

 greater value on the satellite than on its planet, and find in it an explanation of 

 the worn character of the ancient topography which is not evident in the newer 

 formations. As we shall see below, this view as to the expansion of rocks may 

 be of value in accounting for certain possibly recurrent as well as accidental recent 

 changes in the shape of structural features on the lunar surface which certain 

 observations appear to indicate. 



There is one rather obscure group of features on the lunar surface which 

 may be immediately due to the expansion of the superficial materials of the crust. 

 These are the numerous slight ridges which intersect the ground and which are 

 fairly visible near the terminator ; these ridges seem to me to be very low, perhaps 

 not more than a score or two feet in height. They are generally rather straight- 

 lined and so placed that they reticulate the level fields in which they lie, dividing 

 them into irregular blocks of very variable area, rarely more than fifty miles across. 

 I have seen what seems the miniature equivalent of this structure, where a sheet 

 of ice on a lakelet has been affected by great changes of temperature, all below 

 the freezing point of water, and has been broken by the expanding process into 

 blocks which, at their contacts, are crushed up into rude little anticlinals, formed 



