824 Transactioxs of the Americaj^ Ixstitute. 



structure must yield ; then the mechanical action and results would 

 be exactly the same which would follow from the yieldinoj of a dome 

 of similar construction upon our earth. The force exerted would be 

 outward, in a direct line from the center to every point of its rim, 

 and of course the sum of all this force, by its reaction, would be 

 directed back to the center; therefore the first point to yield must be 

 at one of those localities ; and when a fracture is once made the 

 immediate ruin of the whole structure will follow, because the sus- 

 tained weight becomes at once unequally divided, and forces are 

 instantaneously developed which direct themselves with sudden vio- 

 lence to every point upon which the dome had previously depended 

 for support, and consequently, the evidences of the catastrophe will 

 be most conspicuous at the center and the rim. The spreading of 

 the dome segment will be a mechanical consequence of its yielding, 

 and this by the mere outward force it exerted, would upheave moun- 

 tain masses all around the rim ; while j,the sudden outward rush of 

 the water to escape at every point, would greatly add to the forces 

 developed by the falling dome, and the result would be the formation 

 of the rugged ring mountains which inclose the larger lunar basins. 



The sudden action of the forces engaged would also be felt with 

 corresponding energy at the center, because all the force which 

 upheaved the rim would by its reaction be directed to the center, 

 where a fracture would also occur, and the fragments also be 

 upheaved, both from the natural action of the sinking dome and the 

 energy exerted in an upward direction by the waters from below, 

 upon the surface of which rested the enormous weight of the crust it 

 was sustaining; the result of which combined action would of course 

 be tlie center mountain common to lunar basins; and as the superin- 

 cumbent earth was gradually lowered to the bottom, fresh fragments 

 would be broken off around the rim, just as we see ice breaking from 

 similar causes ; this would leave the terraced formations to be found 

 upon the interior slope of the ring mountains, while the convex floor 

 so frequently met with is but a slight modification of the upper sur- 

 face of the dome as it existed before its fracture. As it sank grad- 

 ually down upon or through the sustaining waters until it found a 

 more solid resting place upon the bottom, its characteristic form 

 would be retained, being supported from below by the plastic sedir 

 ment or clay filling up the concavity of its lower surface. 



Kow, if it so happen that the body of water which produced the struc- 

 ture is not in communication withsourcesof constant supply from the 



