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



the average outer slope, according to my estimates, exceeding forty degrees, the 

 inner slope being generally somewhat steeper. On this hypothesis this inner slope 

 must mark the path of the impinging bolide, and the cone that surrounds it be the 

 result of the outthrusting action of that body, such as we note when a pebble is 

 thrown into soft clay or a shot from a cannon enters an armor plate. We have 

 under Gilbert's hypothesis to suppose that the impinging bodies came into con- 

 tact with the moon at something like planetary velocity. Such bodies having a 

 diameter of even a mile and some of them must, on this hypothesis, have been of 

 fifty or more miles diameter would, by the conversion of their momentum into 

 heat, have served to melt a wide field of the crust about their points of contact. 1 

 As there is no trace of any such bolides in the bottoms of these craters, but com- 

 monly a floor, as of hardened lava, we have to suppose that they penetrated to a 

 great depth and that the lava flowed up after their entrance. But the necessary 

 effect of the entrance of a mass sufficiently large to have punctured these open- 

 ings would, if they had penetrated to a molten zone, have been to send up a 

 quantity of lava far more than sufficient to fill the opening they made, while in 

 fact with few, if any, exceptions, this lava appears at no time to have risen to the 

 general level of the surrounding rampart. Furthermore, if the cones about the 

 craters were due to outthrusts caused by such impacts on material stiff enough to 

 maintain the steep walls of the crater, then we should have evidence of radial 

 cracking in the form of open rents, such as would inevitably be developed under 

 the assumed conditions, but have evidently not produced in far the greater 

 number of the vulcanoids. 



There is another and, taken alone, conclusive argument against the suppo- 

 sition that the lunar craters are due to the impact of bolides ; this is found in the 

 facts presented in the series which may be traced in the sizes and distribution of 

 the fractures which it seeks to explain. As regards their sizes, the pits grade 

 from the smallest that can be discerned by the most powerful telescope, probably 

 not over five hundred feet in diameter, to rings that are one hundred miles across. 

 The steepness of the inner slopes of these cavities does not perceptibly differ, nor 

 is there more evidence of lava having been poured out from the larger than from 

 the smaller craters. Moreover, there is no better evidence of radiating fractures 

 in the case of the larger than in the smaller pits. Furthermore, there is no 

 such relation in the masses of material composing the enveloping cones or rings 

 as we would expect to find if they were due to the impact of bodies varying in 

 size as we have to suppose. In many instances the walls of a pit scores of miles 

 in diameter are no thicker or higher than in the case of other pits less than a 

 mile across. 



As regards their distribution, the craters of the moon are generally placed 

 in such apparent lack of order as to give some warrant for the hypothesis that 



'Assuming that the impinging body came upon the surface of the moon at planetary velocity, 

 and that all the resulting heat was applied to its mass, the resulting temperature would exceed, 

 according to my reckoning, 150,000 degrees. A bolide fifty miles in diameter would be likely to 

 melt an area many times its diameter. 



