94 THE GREAT LUNAR CRATER TYCHO. 



sary to solidity liquid lava at a t(Miiperatme equivalent to that at which 

 it is solidified beneath the earth's surface, and any change of pressure 

 that releases a stratum of rock from the solid to the liquid state would 

 upon the moon release a stratum approximately six times as thick, 

 other conditions being similar, and would presumably give rise to lava 

 flows on a gigantic scale compared with terrestrial evolutions. Added 

 to these considerations, we must remember that under the feeble action 

 of lunar gravity crater rings and cliffs may be built up of similar mate- 

 rial much more steeply upon the moon than at the earth's surface. 



There are many formations upon the moon which do not take the 

 form of crater rings. The Riphrean Mountains as shown in a photo- 

 graph taken by the Brothers Henry in May, 1890, is a very good 

 instance to cite. I should like to draw special attention to a curious 

 straight black streak between the crater's Birt and Thebit. It is spoken 

 of by Webb as a wall, but it rather seems to be a narrow valley or 

 fault. It is shown on several of the photographs taken by the Brothers 

 Henry. I would also draw the reader's attention to three dark spots 

 on the floor of the crater Alphousus, which are shown in all good pho- 

 tographs, as well as the curious marking, like a capital G, near to the 

 centre of the Mare Nubian. 



