THE GREAT LUNAR CRATER TYCHO. 93 



theory which professes to accouut for craterlets must account for the 

 types of crater into which they gradually merge. We therefore seem 

 driven back to the volcanic hypothesis, and have to exi)lnin why upon 

 the moon, which is so much smaller than the earth, the volcanic out- 

 breaks have been on so colossal a scale. We are not even in a position 

 to say that the moon is made of similar materials to the earth — indeed, 

 we know that its average density is considerably less, the earth being 

 about 5*66 times as heavy as a similar globe of water, while the moon 

 is only about 3-39 times as dense as water, or, according to Dr. Gill's 

 recent determination, about 1 per cent less. We must not however 

 conclude from this difference that the moon is made of different mate- 

 rials from the earth, for we know too little as to the behavior of solids 

 under the enormous pressures that they must be subjected to at even 

 a few miles beneath the earth's surface. The average density of the 

 rocks of which the earth's surface is composed is only about two and 

 a half times that of water, but it does not follow that the central parts 

 of the earth are comjiosed of different and heavier material. The great 

 rigidity of the earth under the tidal strains iuiposedupou it by the sun 

 and moon points to the conclusion that the solid materials of which the 

 earth is built up are rendered rigid by compression, and that the idea 

 of a fluid interior must be abandoned. Mr. George F. Becker, of the 

 U. S. Geological Survey, has recently pointed out that the slags, into 

 which most of the stratified rocks of the earth's surface would be 

 reduced by melting, increase in bulk on fusion, and are not like iron 

 and water, w4iich expand on solidifying; consequently, he argues that 

 any crust which formed on the surface of a molten sphere of slag would 

 speedily break up by its own weight and sink, and that the process 

 would go on until the whole mass had been reduced in temperature by 

 such upheavals to near the melting point of the slag. But if the liquid 

 slag, or other materials of which the earth is composed, were capable 

 of being reduced by the pressure of the superincumbent mass to the 

 solid condition, such upheavals would not take place, and under such 

 circumstances it is jjossible that the heat of the earth may go on increas- 

 ing to its center. 



If below the surface of the earth large masses of highly heated rock 

 are kept solid by the enormous pressure of overlying rocks, earth move- 

 ments, caused by the cooling and contraction which crumple up the 

 stratified rocks of the surface and give rise to the upheaving of moun- 

 tain chains, may occasionally take off some of the weight from the 

 rocks beneath, causing the highly heated rocks to run off into the 

 liquid state again and find their way to the surface, causing the phe- 

 nomena we know as volcanic action. 



If we adopt this theory of volcanic action, and assume that the moon 

 is made of similar materials to the earth, we shall — Avith lunar gravity 

 only equal to one sixth of terrestrial gravity — need to pass to a depth 

 six times as great upon the moon in order to obtain the pressure neces- 



