A COMPARISON OK THE FEATURES OF THE EARTH AXH THE MOO\. 67 



evidently lava fields, save in origin, essentially like those within the larger vulca- 

 noids, there is evidently no place where waters in a sufficient extent to supply 

 rainfall could have been stored. In this connection it is worth while to note that 

 on the earth, with two-thirds of the surface covered with water and with air 

 currents to carry moisture, large areas are practically unsupplied with water. 

 Without the oceans it is evident that rainfall would cease. The little which is 

 evaporated from the land would readily be stored in the air, perhaps to fall as 

 dew. So that lunar rains or snows would be impossible without a system of 

 great reservoirs, such as we cannot believe to have existed in any recorded stage 

 of the moon's history. 



There remains but one agent of erosion which can have acted on the moon, 

 i. e., that arising from the expansion and contraction of rocks in the changes of 

 temperature which there occur. On the surface of the earth, where the average 

 annual variation of heat on rock faces does not exceed about twenty degrees 

 Centigrade, and where the maximum variation is probably not more than fifty 

 degrees Centigrade, the effect of the variations is evident. Excluding, as far 

 as we may, the concomitant influence of freezing water, we find that the expansion 

 of rock is competent to produce cracks and to urge detached masses of rock down 

 the slope on which they lie. Thus the concentric structure which develops near 

 the surface in certain crystalline rocks, as granite, is due to the expansion of sum- 

 mer heat, which often causes the slabs of stone sensibly to lift from their bed.s. 

 On the surface of the moon, according to Langley's observations, the rano-e of 

 temperature is probably not less than two hundred degrees Centigrade, so that the 

 measure of expansion and contraction should be fourfold what it is on the earth. 

 Moreover, these alterations of temperature are repeated each month. Durino- the 

 fourteen days' insolation, the heat should effectively penetrate for some meters of 

 depth. Though it is doubtful if the melting point of water is ever attained, the 

 range is as effective in promoting motion as if it occurred above that point. 



The effect of the great alterations of temperature in the superficial materials 

 of the moon is probably twofold ; in the firmly imbedded rocks it must institute 

 successive strains and releases which should be competent to produce certain 

 effects not recognizable on this planet. Supposing that at a depth of three meters 

 the range of temperature was one hundred degrees Centigrade, the horizontal 

 thrust induced, if the rock had the modules of expansion of ordinary granite, would 

 be sufificient to produce in a sheet fifty miles in diameter an extension of some 

 hundred feet. From what we see of like action on the surface of the earth, we are 

 justified in supposing that sheets of great width would on the declivities of the 

 moon become separated from the subjacent materials and move over them in the 

 alternations of volume. So, too, we may suppose an interminable series of 

 varying adjustments which would, from time to time, bring about alterations in 

 the direction and energy of the thrusts which were thus induced. These changes 

 may have continued throughout a period as long as recorded geologic time, and 

 they may be in process of development to-day. 



Another consequence of the variation in bulk of rocks in the changes of 



