February, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



27 



Fea^tvires of the CoLrth 

 8Lnd Moon.. 



Volca-rvic FormaLtions. 



that matter expelled from a crater vent could be thrown 

 very much farther or very much wider than is the case on 

 this planet. But, says Professor Pickering, we are really 



Some fascinating speculations on the formation of the 

 Earth's crust emerge from the publication among the 

 Memoirs of the American Academy of Professor W. H. 

 Pickering's " Lunar and Hawaiian Physical Features." 

 The physical features of the island of Hawaii which Pro- 

 fessor Pickering has photographed and investigated are 

 its huge craters and lava plains and lakes ; and he has 

 noted a similarity m their formation and contours with 

 the very much larger craters of the Moon. The Earth 

 and the Moon being supposed to have a common origin, 

 and tj have once been part of the same mass, it would 

 appear not unreasonable to expect that their exterior 

 features would bear some resemblance. But as Professor 

 Pickering observes " the lunar surface presents such a 

 strong contrast to the more thickly populated portions of 

 the Earth that little resemblance between them can be 

 traced. Even those of our volcanic regions which have 

 been most extensively studied show little analogy to the 

 Moon." 



The only Earth cra'ers which bear a colourable re- 

 semblance to those of the Moon are the craters of Hawaii, 

 with which Professor Pickering in the work before us 

 compares them ; and from a consideration of the 

 Hawaiian craters his theory of the reason for the differ- 

 ence between lunar and terrestrial formations, is chiefly 

 drawn. The enormous discrepancy in size between 

 the craters of the Moon and those of the Earth is some- 

 times attributed to the fact that the force of gravitation 

 at the surface of the Moon is but one-sixth as great as it 

 is on the Earth. But this theory will not do ; although 

 if the lunar craters had been due to explosions of steam, 

 as the Earth's explosive volcanoes are, one may grant 



Pjg. 2.— Kies and Mercator. 



trying to compare objects formed under 

 conditions. " The larger craters on the 

 existence when the thin solid crust cov 

 interior was, owing to the solidification 



HiK. 1. Interiur ul Itulcnkald. 



entirely difTerent 

 Moon came into 

 ering the molten 

 and contraction 

 of the crust, much 

 too small to con- 

 tain the liquid 

 material. The 

 craters we re 

 therefore formed 

 by the lava burst- 

 ing through the 

 crust, and so re- 

 lieving the press- 

 ure." In other 

 words they were 

 overflow erup- 

 tions. 



Then, at a se- 

 cond period in the 

 Moon's history, 

 another form of 

 crater, or rather 

 another form of 

 volcaniceruption, 

 came into play. 

 Tiie crust of the 

 Mixin had thick- 

 ened ; and the in- 

 terior regions, by 

 cooling, shrank 

 away from the 

 solid shell, as a 

 drying walnut 

 does. The solid 



