A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 39 



that these bleb-like elevations may have been the first stage of such vents ; in 

 other cases the cones appear to pass by a series of transitions into the mountain- 

 ous form. I have not been able to verify this passage from the dome to 

 the peak, but the indications of it appear to me to be noteworthy. In this 

 connection it may be remarked that the structures in the centers of the middle- 

 sized vulcanoids lend support to the view that a dome-shaped elevation may, by 

 further development, pass into a peak. When these prominences are low and 

 small they often have a rather evenly arched form, but when they are of consid- 

 erable magnitude they take on a complicated shape with serrate crests substan- 

 tially like the structures classed as mountains, the only evident difference being 

 that the masses are not so commonly elongate in horizontal section, as the indi- 

 vidual mountainous ridges commonly are. 



The observed facts concerning the mountainous protuberances of the lunar 

 surface lead me to the opinion that they are classifiable in one group, of which 

 the simplest and most interpretable examples are found in such peaks on vul- 

 canoid lava plains as that of Theophilus, where we have a mass of ejected ma- 

 terials which shows no trace of flowing for it has very steep walls. (See plate 

 xvn.) This great viscid ejection covers an area of more than three hundred 

 square miles, and rises to -a height of six or seven thousand feet above the floor 

 of the crater ; it is particularly interesting for the reason that while it is essen- 

 tially a group of peaks it retains traces of what seems to be a volcanic type, as it 

 has an indistinct crater on the summit of the mass. Other instances could be 

 cited to show this passage from the conditions of a crateriform structure to a 

 rugged cone. In fact the series appears to be sufficiently fairly complete to 

 establish the point that the last stage of activity in the craters of the vulcanoids 

 was that in which the interior lavas, primarily hot enough to flow in the manner 

 necessary to form very level surfaces, had become so viscous that they would 

 maintain themselves at angles of sixty degrees or more to the horizontal. 



As for the ejections of viscous lava which took place outside of the craters, 

 forming mountain-like elevations, the evidence appears to warrant the conclusion 

 that they represent, as do the craterless cones within the rings, a survival of a 

 tendency to eruptions after the time when the lava was liquid enough to produce 

 the normal vulcanoid structures. In these later eruptions, because of this ex- 

 ceeding viscosity of the ejected material, there could be no ring wall or interior 

 lava plain formed. All the material which would have gone to such construc- 

 tions was heaped in the viscid mass which was forced out of the opening. The 

 natural result of these conditions is that the mountainous elevations, while less in 

 diameter than the larger vulcanoids and having no more material than goes to 

 the formation of an ordinary lunar cone and lava plain, present normally very 

 elevated peaks. 



It may seem that if the craters and the mountains are the result of es- 

 sentially the same expulsive energy, with no other difference in the conditions 

 than the suggested variation in the fluidity of the lavas, we should find a series 

 of intermediate forms between the crater and the peak. Such intermediate 



