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



sufficient reasons for rejecting the suggestion that they are due to the impact of 

 falling bodies. 



In addition to the features of the lunar volcanoes there is another though 

 more remote reason why such falls of celestial bodies on the moon's surface have 

 not occurred. Of these we may here mention two ; these are as follows : It is 

 evident that these vulcanoids were formed at successive times, and under some- 

 what diverse conditions. So far as I have been able to determine, the largest 

 were, at least in a general way, first produced, and the smaller, approximately, in 

 the order of diminishing size, the smallest in most instances being formed last. 

 Now, as will be more particularly noted hereafter, the light bands which radiate 

 from certain craters and which are clearly mere strips of material which at full 

 moon reflect the sun's light more intensely than the general surface have evidently 

 not been covered by deposits of ordinary meteoric matter, such as falls on the 

 earth in considerable quantity. It thus appears that for some reason the moon, 

 provided its surface has anything like the antiquity it appears necessary to assign 

 to it, has not been the seat of such deposits ; for the accumulation of a small 

 amount of meteoric matter would mask such stains. We would thus, according 

 to the Gilbert hypothesis, have to suppose a succession of showers, each sending 

 bolides of smaller size than the preceding, and with them no considerable amount 

 of ordinary finely divided meteoric material such as comes to the earth. 



It is also to be noted that since the earth's surface came to its present state 

 there is good reason to believe that no such falls of large bodies as are supposed 

 by the bolide hypothesis to have fallen upon the satellite have ever come to the 

 planet. There are no traces of like craters, for even the greatest calderas, such 

 as that which holds Lago Bolsena or Kilauea, are evidently volcanic and in no 

 way related to meteoric action. Moreover, the fall of a bolide of even ten miles 

 in diameter would, by the inevitable development of heat due to its arrest, have 

 been sufficient to destroy the organic life of the earth, yet this life has evidently 

 been continued without interruption since before the Cambrian time. The point 

 to be last noted is that so far as I have been able to determine from an extended 

 inspection of lunar craters, including several hundred of the more determinable, 

 they all have the a.xes of their pits at right angles to the surface. Now if these 

 pits had been formed by bolides encountering the moon in their movement, that 

 movement necessarily being at planetary velocity, it does not seem possible that 

 they could all have come upon the sphere in a path normal to its surface. Even 

 with the resistance of the earth's atmosphere, which is far denser than that of the 

 moon ever could have been, the small meteors which enter it mostly come at high 

 angles to the surface of the planet, although its attractive power is more than 

 eighty times as great as that of the satellite. It seems, indeed, incredible that if 

 the lunar vulcanoids were due to bolides they should not have fallen in some- 

 what greater numbers on the earth because of its o-reater grravitative attraction. 

 The number received would probably be nearly in proportion to the area of the 

 two spheres, with a slight preponderance in the number falling on the earth 

 because of its greater mass and consec^uently the greater effect of its gravity. It 



