28 A COMPARISON OF THK FEATURES OF THE EARTH AND THE MOON. 



they are — mainly if not altogether — the points of discharge of water-vapor or of its 

 dissociated gases, and that this water has been buried by aqueous sedimentation. 

 The result is that when heated to a high temperature the fluid commonly explodes 

 with a great tension, scattering large amounts of morcellated rock to great 

 distances from the place of escape. On the other hand, in the lunar vulcanoids, 

 the evidence goes to show that there were no explosions competent to drive 

 fragments in extended trajectories. It is evident, indeed, that the movement of 

 the lava in the pits was almost exclusively up and down in the cavities, often with 

 successive haltings on a particular level, followed by a sinking to a considerable 

 depth. In these stationary periods, the terraces of the frozen fluid on the inner 

 slopes of the ramparts apparently were formed. That the position of the lava 

 was not in all instances determined by a common interior deep level of the fluid 

 seems to be shown by the fact that in some of the rings its surface is several thou- 

 sand feet below the surrounding area, while in the case of Wargentin, just south 

 of Schickard, the floor apparently lies high above the surface of the surrounding 

 country. 



That there was some kind of boiling or up-welling action in these crater lavas 

 is well shown by the fact that in a number of instances, more numerous than the 

 records show, the surface of the floor is flexed upward, so that the center is some 

 hundred feet above the rim of the sheet, as if the final much weakened impulse 

 was sufficient to arch the frozen crust but not great enough to rend it from 

 its adhesions to the shore. Such tumefying action is also shown by the numerous 

 instances in which a mountainous mass of lava has been forced up in the central 

 part of the crater floor. These medial heaps of lava are so common in the 

 vulcanoids of middle size as to be the rule rather than the exception in these 

 structures. In many instances they are replaced by central craters, or now and 

 then, as in the case of Theophilus, there is a mass spewed up, as are some terres- 

 trial trachytic cones, with only a faint trace of crater pipes leading downward into 

 the interior. (See plate xvii.) 



Finding as we do evidence of some swelling and sinking process competent 

 to lift and lower the lava in the craters of the vulcanoids, and seeing at the same 

 time that this action did not take place with anything like the energy of terrestrial 

 eruptions, the question arises as to the nature of this eruptive force which has 

 operated on the crust of the moon. The only hypothesis which has suggested 

 itself is some kind of boiling, such as will take place in any fluid mass which is 

 heated below and cooled on the surface, as in molten iron, where substances in 

 the vaporous state, though they exist, are not present in sufficient quantities 

 greatly to affect the movement, or there is a circulation mainly impelled by the 

 escape of imprisoned vapors. Mere convection of heat in an igneous fluid 

 does not seem to be sufficient to account for the rise and fall of the lava in the 

 craters, especially as in the case of Wargentin, for there the lava floor lies at 

 a height of some thousands of feet above the general level of the surface. We 

 will therefore consider the possibility of there being materials vaporized by heat 

 in the lava, not enough to produce the type of terrestrial explosions, but sufficient 



