1 8 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



It may further be said in favor of this hypothesis as to the origin of the maria, 

 that the material of which they are composed appears to have had throughout 

 the whole extent of the several areas a singularly uniform fluidity. As before 

 remarked, there are no signs of successive flows such as have always characterized 

 the accumulation of the relatively much less extensive lava deposits on the sur- 

 face of the earth. In this connection it should again be noted that none of the 

 vulcanoids show any tendency to send forth extended flows, and the matter which 

 appears to have been ejected to form the cones has evidently consolidated on 

 very steep slopes. Thus, if the material of the maria was fluid when it came to 

 rest, of which there seems no reason to doubt, it cannot have been poured forth 

 from the interior in the manner of volcanic effusions. 



The fact that the surfaces of the maria are of a distinctly darker color than 

 the other and higher extended areas of the moon has some value as evidence that 

 they have a peculiar origin, one not connected with the interior of the sphere. 

 Certain of the crater floors have, it is true, about the same tint ; this is con- 

 spicuously the case with Plato. In this, as in certain other instances, the like- 

 ness may be due to the penetration by subterranean passages of the material of 

 the neighboring mare into the cavities of the craters. There are, however, exam- 

 ples, as, for instance, the great vulcanoid Grimaldi, where the resemblance 

 cannot be thus explained. Although these exceptions weaken the value of this 

 evidence derived from the color of the maria, the uniformity of a tint which is 

 evident in all of them and the seldomness of the exceptions tend to support the 

 hypothesis that the rocks of which they are composed have not come from the 

 interior of the sphere. This point will be further discussed below. 



We turn now to consider the objections which may be made to the hypothe- 

 sis that the maria were formed by molten rock produced by the impact of large 

 bodies falling upon the surface of the moon. Of these objections, the first and, 

 in many regards, the strongest is derived from the general consideration that like 

 bodies competent to generate a great deal of heat have not fallen upon the 

 earth's surface in the time which has elapsed since the beginning of the geological 

 periods. There is indeed no geological reason for supposing that they have ever 

 so fallen upon the planet. 



Against the above-noted objection that the geological record of our sphere 

 affords no trace of evidence of any such falling-in upon its surface of bodies of 

 sufficient mass to produce widespread melting, and the proof that no cataclysms 

 of this nature have occurred since the development of organic life, we may set 

 the following considerations : first, that the moon's surface probably took its 

 shape long before the beginning of our geological record ; and, second, that even 

 in this late stage in the evolution of our solar system there remain bodies in that 

 system in order of size such as would in falling upon the surface of the larger 

 spheres produce the effect which we observe in the maria. Thus the group of 

 asteroids which lie between Mars and Jupiter, though generally of far greater 

 mass than would be required by impact to melt the larger of the mare fields, 

 probably contains many bodies which, in case of collision with our satellite, 



