302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



continued an evolution of highest life. Is the earth striving to at- 

 tain a final uniformity or radius, and, provided the solar energy is 

 of sufficient duration, will a planetary history which began in a 

 rejgn of fire end in the establishment of a universal ocean ? Science 

 as yet can not predict, for the end is hidden behind a deeper veil 

 than that which conceals the beginning. 



THE GEOLOGIC BEARING OF THE MOONS SURFICIAL FEATURES 



In studying the earth as a planet, we labor under the difficulty of 

 observing only one example of a class. If the earth can be compared 

 to a similar body, tests may be found that make more probable the 

 hypothesis framed to explain the features and history of our globe. 

 Fortunately the moon is such a body, and one so near that under 

 high powers of the telescope features 500 feet in diameter may be dis- 

 tinguished. Geologists, their eyes turned toward earth, have sought 

 but little knowledge in the heavens, though Gilbert, Suess, Shajer, 

 and others have studied the moon with the vision of the earth scien- 

 tist. Nevertheless, the features of our satellite reflect something 

 of the history of the earth, and in particular support the hypothe- 

 ses of the internal origin of the hydrosphere and atmosphere, as well 

 as give evidence pointing toward the periodic growth of ocean basins 

 by continental fragmentation. As throwing light upon such geologic 

 problems of genesis, these features of the moon are consequently 

 worthy of some detailed discussion. 



The moon, at a mean distance of 238,840 miles, is by far the near- 

 est celestial body. Its diameter, 2,163 miles, is but 27 per cent of that 

 of the earth, and yet this is by far the largest ratio of any satellite 

 to its primary in the solar system. In fact, the size of the moon 

 really removes it from the class of satellites into the unique posi- 

 tion of a small sister planet. The mean specific gravity of the earth 

 is 5.53; that of the moon is 3.34. Consequently the mass of the moon 

 is but 0.012 that of the earth, and gravity at its surface is but one- 

 sixth of the same force. at the surface of the earth. The earth's 

 sphere of gravitative control extends outward to somewhat over 

 600,000 miles, far beyond the orbit of the moon. The gravity of the 

 moon, on the other hand, is not sufficient to hold an atmosphere to 

 itself. Such gases as emanate from its interior diffuse into space, 

 and being within the earth's sphere of control have been in greater 

 part added to the air and ocean of the earth. The moon has there- 

 fore never had seas, nor has it been subject to the gradational effects 

 of air and water. It preserves in the rich detail of its surface features 

 a record of its internal activities, a record similar to yet different 

 from that which has been obscured on the earth by concealment be- 

 neath the sedimentary and oceanic envelopes or by destruction 

 through the gradational forces of the atmosphere. 



