40 A COMPARISON'- OF THE FEATURES OF THE EARTH AND THE MOON. 



Stages are, as I have noted, to be found in the central structures of the normal 

 vulcanoids. I have here and there suspected like transitional shapes among the 

 mountains, but can cite none that is conclusive. There are, however, in the 

 region of the Alps and other fields in the northern hemisphere of the moon vari- 

 ous instances which may be of this nature. My eyes are no longer fit for such 

 difficult observations, so I must leave this point, along with many others, unveri- 

 fied. It is well, however, to note that the passage from the state in which the 

 lava of the moon's interior was sufficiently fluid to bring about the formation of 

 the ordinary vulcanoids, to that in which peaks only would be formed, does not 

 involve any great change of temperature. In terrestrial conditions, a lowering of 

 a few degrees in heat at the critical point in a progressive cooling would be suffi- 

 cient to bring about the change in the nature of the eruption. 



The frequently elongate shape of an individual mountain seems at first sight 

 to be, and perhaps really is, an objection to the above-described theory of their 

 origin. It is, however, to be remarked that a large part of these elevations have 

 rudely circular bases, and that where they depart from this figure they do not 

 take the shape of long, continuous ridges, the major axis rarely exceeding the 

 minor in the ratio of more than two to one ; moreover, some of the mountains 

 of the crater floors show the same tendency to elongation. Later on in this 

 writing I shall note that the phenomena of "rills" and other rifts show that the 

 surface of the moon was very generally in a state of contractile tension, and this 

 before the formation of the smaller vulcanoids was arrested, and further that the 

 axis of the mountains often coincides with the direction of the rill-splitting. If 

 this be the case, then the extrusion of somewhat rigid materials such as formed 

 these cones would naturally tend to rend the crust as with a wedge, so that 

 an elongated opening would be formed for the extruded mass and the shape of 

 such opening would determine the outline of the elevation. 



There is yet another class of reliefs on the lunar surface, those which are typi- 

 fied by the great escarpment of the Altai Mountains in the fourth quadrant. (See 

 plate XVI.) In this Altai relief we find in the southeast a slight and gentle rise of 

 a field, which has few very noteworthy features, for a hundred miles or more to the 

 edge of the steep, and then a sudden fall to the northwest, the descent being on 

 the average at least six thousand feet. The crest of this declivity is much varied ; 

 one peak, at least, is said to attain the height of thirteen thousand feet above its 

 base. It appears likely that the northwest face of the Haemus Mountains and 

 the southeast face of the somewhat similar district lying between Eratosthenes 

 and Mt. Hadley, facing the Mare Imbrium, are structures of a related nature. 

 The most warrantable hypothesis, from the point of view of the geologist, is that 

 these reliefs are due to faulting on a large scale, accompanied by a considerable 

 amount of extrusion of the type that forms lunar peaks. In two of the three 

 evident examples of this group, those last named, the lava of the maria has 

 extended to the base of the declivity ; in the case of the Altai steep, the igneous 

 matter of the Mare Crisium, though it once extended much beyond its present 

 limits, did not attain the base of the escarpment. There are divers other steeps 



