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



ever, there is a striking difference in the behavior of lunar and terrestrial lavas ; 

 the lunar, except in the maria, where the evidence of high and continued liquidity 

 seems to me plain, appear to have become stiff almost as soon as they escaped 

 from their craters, a fact which may be accounted for by their viscidity or perhaps 

 by the swift cooling to which they were exposed on;the airless sphere. 



It is noteworthy that the most important differences of hue on the lunar 

 surface are found in the maria and certain of the greater vulcanoids. The maria 

 are without exception much darker than the higher ground. The lavas within 

 the craters are likewise rather dark, but less conspicuously so ; but in the case of 

 certain of the great rings near the eastern limb, notably Grimaldi, they are quite as 

 somber hued as any of the seas. In the neighboring vulcanoid, Riccoli, there is a 

 patch on the floor which is perhaps the darkest-colored of any part of the lunar 

 surface. If these dark lavas were altogether peculiar to the maria, it would be 

 easy to account for their color by the supposition that the material imported by 

 the bolides, which I have supposed to have caused their formation, was of a dif- 

 ferent constitution from the materials of which the general surface of the moon is 

 composed. The frequent incoming to the earth of considerable meteoric masses, 

 composed in large part of iron, would warrant the hypothesis that the bolides 

 which produced the lavas of the maria were largely made up of this metal. Even 

 if not the tenth part of the lavas were of this foreign material, it might serve to 

 effect the'.darkening of the resulting sheet. The occurrence of a like hue in lavas 

 which lie on the central floors of distinct vulcanoids appears to negative this 

 supposition. 



Although for the reasons given above I cannot at present strongly maintain 

 the hypothesis that the hue of the seas is due to the color-producing action of 

 the bolides which produced them, it is perhaps hasty to dismiss the view without 

 some consideration. It may be urged that the in-falling bodies were probably of 

 varied sizes. Thus the mass or masses which I have supposed to have produced 

 the isolated Mare Crisium were probably smaller than the mass or masses which 

 brought about the formation of the great system of connected maria. It is fairly 

 supposable that a fragment large enough to have given the lava of Grimaldi its 

 peculiar hue fell within that vulcanoid, and that a small fragment likewise affected 

 a part of the floor of Riccoli. So numerous and crowded are these great vul- 

 canoids near the eastern limb of the moon that there is more than an even chance 

 that two such falls would both lodge within some of them and not in the inter- 

 vening country. As before noted, Plato and other less conspicuous vulcanoids 

 situated near the maria have dark floors, but in these cases there is a fair chance 

 that the external bodies of lava may, while fluid, have penetrated into their 

 enclosures ; its evident exceeding fluidity would probably enable it to burrow its 

 way in, though the more viscid lavas of the craters in no case appear to have 

 been able to flow out through the cones. 



Thus while the facts do not warrant us in concluding that the color of the 

 lavas in the maria is due to mineral peculiarities imported by bolides which formed 

 them, it strongly suggests that explanation. Progress towards an interpretation 



