254 
GILBERT. 
the flood at its edges, so as to mark its limit by a solid ridge. 
By each successive tide the operation was repeated, with 
the result that the wall was given a circular form and was 
gradually built up. The process was finally closed by the 
congelation of lava in the orifice, and while congelation 
was in progress the last feeble eruption sometimes produced 
a central hill. 
In certain respects this theory is well founded. It is true 
that the earth is able to produce far greater tides on the 
moon than the moon produces on the earth, and if we may 
accept the conclusion of G. H. Darwin that the moon is 
retreating from the earth,* then the reciprocal tides of moon 
and earth were greater at an earlier date than they are now. 
That a circular ridge may be built up by the alternate ex¬ 
trusion and retraction of a suitable substance through an 
orifice has been demonstrated by Ebert, who devised appa¬ 
ratus and conducted a series of experiments. The crater 
rims he achieved sloped regularly outward and were steep 
and rudely terraced inward, thus reproducing the more im¬ 
portant features of the lunar rims, with the exception of the 
wreath, and by special manipulation he was able to approach 
the characters of the wreath. 
In other respects the theory finds less support. At the 
time of formation of the larger craters the crust must have 
been thick and strong to sustain the weight of their rims. 
It could not then or afterward have been divided by a close 
plexus of cracks, but such a plexus seems necessary under 
the theory to account for the multitude of small craters 
which overlie the large. Again, it is pertinent to inquire 
whether the crustal strains engendered by great tides in a 
liquid nucleus would find relief in the postulated manner. 
If the crust were divided by fissures, would not the tensile 
strains wrought by the crest of the tidal wave cause the 
fissures to gape, instead of forcing out the liquid through 
apertures here and there? Or, if there were no fissures, 
would not the strains suffice to produce them? The postu- 
*Article “ Tides ” in Encj'cl. Britannica, vol. 23, p. 378. 
