FRAGMENTATION— BAREELL 303 



If, in imagination, the oceans should be removed from the earth 

 and the planet were to be viewed from outer space, one of its most 

 .striking features would be the systems of mountain folds. These 

 bear the evidence of being made by great compressive movements 

 in the crust, and imply a condensation of the earth's body operat- 

 ing periodically throughout geologic time. This crustal wrinkling 

 is clearly not due to external cooling, as Dutton, Fisher, and Cham- 

 berlin have shown. Chamberlin has given cogent reasons for hold- 

 ing that it is indirectly due to a condensation of the nucleus of the 

 earth, and that the direct cause may be found in the enormous pres- 

 sure acting thereon, due to the weight of the lithosphere. This 

 pressure attains 45,000,000 pounds per square inch at the earth's 

 center, a depth of 3,959 miles. The moon's center at a depth of 

 1,081 miles is subjected to only a small fraction of this pressure, 

 equivalent to that which would be found in the earth somewhat 

 less than 100 miles beneath its surface. Thi,s lack of high internal 

 pressures seems, therefore, to explain the absence of important 

 linear mountain systems on the moon. 



The moon's face, as seen especially in the full moon (pi. 1), con- 

 sists in part of a brilliant but rough surface interspersed with 

 coalescent, smoother, darker areas which individually have irregu- 

 larly rounded boundaries. To the naked eye these darker areas 

 combine into the rude semblance of the features of a human face. 

 As Galileo looked at these features through the first telescope, the 

 resemblance of the darker areas to seas so impressed him that he 

 named them " maria," giving to each unit a distinctive name such 

 as "Mare Imbrium" (shown in pi. 3) and "Mare Serenitatis." 

 Closer observation shows that these " seas " are in reality more or 

 less smooth dark lava plains, dotted with occasional volcanic craters. 

 Over the brighter portions, beyond the lava plains, the craters stand 

 very thickly, older craters are more or less obliterated by numerous 

 j^ounger ones superimposed upon them, and the whole constitutes a 

 chaos (shown well in pi. 2). Zollner has computed from the amount 

 of light received at different angles from thi,s surface that the 

 average angle of slope is at 52'' to the horizontal. This corresponds 

 to the reflection from a rough, loose surface of cracked stone or a 

 surface of steep, rough, and vesicular lava. The comparative 

 smoothness and darker color of the maria indicate a comparative 

 absence of these features, resulting in a different light refraction. 



Loewy and Puiseux find that three successive surficial levels may 

 be distinguished in the moon. The first is the rough, brilliant, 

 higher lying region in the southern hemisphere; the second, at least 

 10.000 feet low'er, consists of the floors of the maria; the third, 

 again 10,000 feet deeper, corresponds to the bottoms of the circular 

 craters which have been formed within the maria after their con- 



