A COMrARISON OF THE FEATURES OF THE EARTH AND THE MOON. 29 



to lift the lava to the tops of the existing rings and to produce a circulation suf- 

 ficient to keep the material for a long time in a molten state. On this point we 

 have some direct evidence from the fact that many types of lavas that form dykes, 

 such as granites, are violently forced into rocks of the earth's crust without there 

 being any evidence of vaporous or gaseous materials impelling them ; it is more 

 likely, however, that what we see in the way of eruptions on the moon are the 

 results of extrusions brought about by the pressure of gases originally contained 

 in the fluid mass of the sphere. 



It is commonly assumed that for a long time after any celestial sphere has 

 entered on its fluid state, in passing from its nebulous or fragmentary previous 

 condition, the process of separation of its materials volatilizable at the temper- 

 ature established by the concentration must necessarily go on with the result 

 that some such vulcanoid phenomena as appear on the lunar surface will be 

 likely to occur. It is a fair working hypothesis that every crater-like opening on 

 the moon was formed by the relatively mild outbreak of vapor such as keeps 

 open the terrestrial craters of the Kilauea type ; in such vents there may be 

 vapor enough to induce some movement of the lava, but not enough to cause 

 very great ejections of the fluid. 



It may be assumed that the lava of the moon far more than that of the 

 earth would tend to retain its gases and to form the viscid, slow-moving material 

 known as pumice, which even when near a melting temperature is of a wax-like 

 stiffness. The reason why the blebs of vapor could not separate from the lunar 

 lava as readily as from the fluid rock of our planet is to be found in the relatively 

 slight value of gravitation, which on the surface of the moon is only a little more 

 than one-sixth what it is on the earth. The tendency of bubbles to separate 

 from a fluid depends in large measure on the difference between the weight 

 of the contained vapor and that of the mass in which they lie ; so that it may 

 well be that the lavas of the satellite were on account of their contained vesicules 

 of vapor less fluid and more like pumice than those we have a chance to observe 

 in volcanic action. 



When the lavas were lifted to the edge of the encircling rampart it is evident 

 that they flowed out. That they were in the periods of activity so lifted and 

 discharged is plain from the height of the terraces in many lunar craters, and 

 from the elevation at which the lava floor has remained in the case of Wargentin. 

 The normal well-preserved vulcanoid of sufficient size to permit a study of its 

 features shows, in most instances, buttress-like ridges extending not more than a 

 few miles outwardly from its rim ; these are fairly to be taken as flows which 

 have passed over that rim or through breaches in it. It is to be noted that all of 

 these buttresses have very steep slopes, both in the radial direction from the 

 crater and laterally from the center of the ridge. To those accustomed to the 

 gradual slope of lava streams, such as break forth from the base of volcanic- 

 cones where the angle of declivity is often not more than two or three degrees, 

 the twenty to thirty degrees of inclination of these supposed lunar flows may 

 seem to negative the hypothesis that they can be lava streams. Lyall and 



