9 6 ROUGH WA YS MADE SMOOTH. 



effects of contraction. Thus we can understand why the 

 signs of volcanic action, as distinguished from the action to 

 which mountain-ranges are due, should be far more nume- 

 rous and important on the moon than on the earth. 



I do not, however, in this place enter specially into the 

 consideration of the moon's stage of volcanic activity, be- 

 cause already, in the pages of my Treatise on the Moon 

 (Chapter VI.), I have given a full account of that portion of 

 my present subject. I may make a few remarks, however, 

 on the theory respecting lunar craters touched on in my 

 work on ' The Moon.' I have mentioned the possibility that 

 some among the enormous number of ring-shaped de- 

 pressions which are seen on the moon's surface may have 

 been the result of meteoric downfalls in long past ages of 

 the moon's history. One or two critics have spoken of this 

 view as though it were too fantastic for serious consideration. 

 Now, though I threw out the opinion merely as a suggestion, 

 distinctly stating that I should not care to maintain it as a 

 theory, and although my own opinion is unfavourable to the 

 supposition that any of the more considerable lunar markings 

 can be explained in the suggested way, yet it is necessary to 

 notice that on the general question whether the moon's 

 surface has been marked or not by meteoric downfalls 

 scarcely any reasonable doubts can be entertained. For, 

 first, we can scarcely question that the moon's surface was 

 for long ages plastic, and though we may not assign to this 

 period nearly so great a length (350 millions of years) as 

 Tyndall following Bischoff assigns to the period when 

 our earth's surface was cooling from a temperature of 

 2000 C. to 200, yet still it must have lasted millions of 

 years ; and, secondly, we cannot doubt that the process of 

 meteoric downfall now going on is not a new thing, but, on 

 the contrary, is rather the final stage of a process which 

 once took place far more actively. Now Prof. Newton has 

 estimated, by a fair estimate of observed facts, that each day 

 on the average 400 millions of meteors fall, of all sizes down 

 to the minutest discernible in a telescope, upon the earth's 



