A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 5! 



in the day-lit sky. The evidence, in a word, goes to show that the surface of the 

 moon is, as a whole, quite as dark as the average lavas of the earth's surface 

 when they are lit by a vertical sun. 



Although the .moon's surface, taken as a whole, must, according to Zollner, 

 be regarded as nearer black than white, there is little doubt that parts of -it 

 under certain conditions of illumination are as white as any portions of the earth's 

 surface ; as white as the chalk cliffs of Dover, probably ; or as white as new-fallen 

 snow would appear to an observer looking upon it from the moon. Although 

 the range in the scale of tint between black and white is probably nearly as great 

 on the moon as upon the earth, it is most noteworthy that there is no distinct 

 trace of the other colors so abundantly exhibited in the terrestrial minerals and 

 rocks. There are no greens or yellows, and it may be doubted if there is any 

 trace of red. Schroter, whose scale of hues ranges from the black shadows to 

 the whitest illuminated objects in the moon, selects ten gradations in that scale, 

 but makes no provision for the prismatic colors ; he evidently did not find them. 

 I have a fair sense of color and have only to confirm this judgment. The geologi- 

 cal importance of this point is considerable, for it clearly indicates uniformity 

 in the lithological composition of the moon, or at least in the aspect of its rocks, 

 which differs widely from that we have on the earth. It appears to me that the 

 value of this uniformity in the color scale of our satellite may fairly be set forth 

 as follows : 



It is a reasonable supposition that the chemical elements of which the moon 

 is composed are essentially like those of the earth, for such identities are indi- 

 cated by the spectroscope in the sun and the remoter stars. It is, indeed, alto- 

 gether likely that all the elements of the terrestrial rocks would be found in 

 those upon the lunar surface. Is there any reason why they should not present 

 us with a like range of color? It seems not improbable that this difference may 

 be due to the lack of water or air on the satellite. In the terrestrial rocks almost 

 all the prismatic colors are due to processes of oxidation which water brings 

 about. Those which are thrown out by volcanoes commonly are without such 

 hues, and only exhibit them when they have been subjected to oxidation on the 

 surface. So subjected, they acquire, by that process acting on various substances, 

 particularly on the iron they contain, a considerable variety;of tint, including yel- 

 lows, blues, and reds. ' Thus it seems to me the lack of color range on the moon 

 confirms the supposition that there neither is nor has been water or free oxygen 

 on its surface. 



Within the range of tints recognizable on the moon we have room for some- 

 thing like as ample a scope of petrographic variation as may be supposed in the 

 varied volcanic rocks of the earth if they were precluded of oxidation. Accord- 

 ing to Proctor, the darker parts of the lunar surface are of the tint which would be 

 reflected by dark syenite. The whiter are probably as bright as the lightest of 

 our volcanic rocks or the encrustations formed by solfataric action. In a word, 

 there is no reason to suppose that the lunar volcanic rocks are any less varied 

 than are those that come from the depths of the earth. As before noted, how- 



