5<D A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



yet certain as to their application to celestial spheres. It is, however, evident 

 that the earth did not cool down to anything like an equal temperature through- 

 out this sphere before a crust was formed. But in the lighter mass of the moon, 

 when gravity tended less to promote interior solidity than it has probably done 

 in the case of the earth, it is possible that boiling went on so long and effectively 

 that when it ceased the whole was at a temperature not much above the heat 

 of lava, so that the further cooling would be uniform, and the undiminished 

 crust would not have in any considerable-measure to conform to the diminished 

 interior. There are difficulties with this hypothesis, as with the others which 

 have been suggested. If we could suppose that the moon had been during its 

 cooling stage deeply wrapped with a vaporous envelope, as was probably the case 

 with our earth at the corresponding stage of its development, it would be easier 

 to conceive a process of slow cooling which would permit the exterior part to 

 attain about the same temperature as the central portion, so that they would 

 solidify at the same time. But it is likely, for reasons given below, that through 

 its whole history as a sphere it has lacked such a covering and has been exposed 

 to the temperature of space. Yet for all these objections it appears probable 

 that the hypothesis last above suggested is the most tenable, and that the greater 

 part or possibly the whole mass of our satellite became solidified at nearly the same 

 time and at nearly the same temperature. 



To the geologist, the action of the lunar surface under the limited com- 

 pressive stresses to which it appears to have been subjected is of especial interest, 

 because it shows clearly that rocks, which certainly are not stratified, apparently 

 may warp into rather sharp up-and-down folds. The student of the earth has come 

 to recognize that, in a limited way, foldings may take place in crystalline rocks 

 where there is no stratification on which the separate parts of the mass may slip, 

 nor even schistose planes that may facilitate such action, but that such extensive 

 and far-reaching movements as are apparently shown in the continuous ridges and 

 furrows of the maria or in the crater valleys may occur, has not been appreciated. 

 So, too, the lunar phenomena suggest to the geologist that the variations in the 

 action of a sphere under conditions other than those now existing on this planet 

 may be exceedingly great. 



DIVERSITIES IN HUE ON THE LUNAR SURFACE. 



Under this head I shall consider the differences in the amount of light and its 

 color which the surface of the moon sends to us, taking first the permanent hue 

 of its several parts and then the variations which occur in the various angles of 

 illumination. Beginning with the observations of Sir John Herschel at the Cape 

 of Good Hope, there have been a number of studies on the light of the moon. 

 Herschel, by comparing the color of the moon with that of the face" of Table 

 Mountain, came to the conclusion that the hue of the satellite did not perceptibly 

 differ from that of weathered sandstone ; that it was rather a dark than a bright 

 object. It is easy to make an equivalent observation when the old moon is seen 



