258 
GILBERT. 
the earth having been obliterated by the processes which 
continually reconstruct and remodel its surface. 
Another difficulty has been found in imagining a condi¬ 
tion of lunar surface which should admit at the same time- 
of plastic molding and of the preservation of the resulting 
forms. The steep inner slopes of lunar craters are, in places,, 
from 15,000 to 20,000 feet in height; their stability in the 
presence of even the feeble gravitational force at the moon’s 
surface demonstrates great strength of material, and the mind 
does not readily associate great strength with plasticity. To 
avoid this difficulty it has been assumed by more than one 
student that the moon’s surface was soft when the craters 
were made; but it seems to me that this assumption does 
not really escape the difficulty, for it will not do to postulate 
a degree of softness incompatible with the survival of lofty 
cliffs. To my mind it appears that the difficulty is only 
imaginary and not real. Rigidity and plasticity are not 
absolute terms but relative, and all solids are in fact both 
rigid and plastic. The apparent contrast between the two 
properties b'elongs to the laboratory and to those phenomena- 
of nature involving small masses and small forces. When 
great masses and great forces are involved, as, for example, in 
the making of continents and mountain chains, the distinc¬ 
tion loses value. The phenomena of mountain structure 
demonstrate that under sufficient strains great bodies of rock 
both bend and flow. If the lunar craters were produced by 
collision, the masses of matter involved were greater than 
those of terrestrial mountain ranges and the concentration 
of energy was correspondingly great. Moreover, a portion 
of this energy may have been converted into heat, with the 
result that the parts affected were rendered less rigid or even 
molten, and it even appears necessary to assume a result of 
this sort in order to account for the level surfaces of the 
inner plain of the craters. My friend, Mr. R. S. Woodward, 
has kindly made for me some computations which serve to 
illustrate this point. If a body fall to the moon’s surface 
from an infinite distance, being influenced only by the- 
