30 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



Others have, however, shown that lavas may, flowing over the edges of terrestrial 

 craters, consolidate in slopes of eighteen degrees of declivity. Now the angle at 

 which the stream comes to rest will, other things being equal, be determined by 

 the value of gravity ; reckoning this as before at one-sixth that of the earth's 

 surface, we see that a very much increased slope may well be allowed in the case 

 of the lunar discharges. 



The conception thus formed of the process by which a lunar vulcanoid 

 of the larger size was produced, a conception founded on an extended study of 

 their phenomena, is as follows : the first stage of the action probably consisted in 

 the production of a slight dome-shaped elevation such as abound on the lunar 

 surface, being, indeed, the commonest of the smaller features on many parts of 

 the areas outside of the maria. These dome-like elevations appear to be due 

 to some accumulation of vapors beneath the superficial layer, formed perhaps 

 when the whole crust was still partly softened by heat. At a certain stage of the 

 process this arch fell in, or was broken to pieces and thrown outwardly, leaving a 

 pit with lava in it. When in its oscillations of height this lava overflowed the 

 edge of the pit, the material so passing from the heated interior quickly consoli- 

 dated and began the formation of a ring-shaped rampart. With the continuance 

 of this action the lava would tend to melt down the interior faces of the rampart, 

 gradually extending the diameter of the opening, destroying and remaking the 

 wall as the process of enlargement went on. Finally, as the supply of melted 

 rock was by unknown causes reduced, the lava fell to its lowest depth and gradu- 

 ally froze ; the last stage in the activity being usually marked by a small central 

 crater, a low dome, or by a spewed-out cone, such as so commonly occupies the 

 central part of the floors of the greater rings. It is to be noted that the present 

 position of the lava in the vulcanoids is not to be taken as its average height, for 

 practically all of the craters which preserve what seems to be a fair semblance of 

 their original form show the remains of terraces that indicate higher levels of 

 their floors. 



The objection may be made that the summits of the ramparts abound in 

 peaks which rise far above the general level of the rings. It is evident that 

 these salient points present serious difficulties ; in some instances they may be 

 accounted for on the supposition that the parts of the ridge now much lower 

 have been broken down by lava which has poured over its crest. In other cases 

 we may find the explanation in the fact that there is an obvious tendency to 

 form small craters on the crust of the ring wall, there being many such that are 

 plainly visible. Now, as we see elsewhere, particularly in the center of the 

 vulcanoids of middle size, sharp, irregularly shaped masses of extruded lava, 

 sometimes, as in Theophilus, many thousand feet high, often take the place of 

 small craters. (See plate xvii.) Thus these isolated peaks may be masses 

 • of lava which have been spewed up to a great height. The origin of the small 

 vulcanoids on the ramparts of the greater is a difficult matter to explain ; it 

 may perhaps be accounted for by reference to terrestrial volcanoes, where we find 

 some evidence of a like tendency to form secondary craters around the margins 



