THE MOON’S FACE; 
A STUDY OF THE ORIGIN OF ITS FEATURES. 
BY 
G. K. Gilbert. 
ADDRESS AS RETIRING PRESIDENT* 
Delivered December 10, 1892. 
The face which the moon turns ever toward us is a 
territory as large as North America, and, on the whole, 
it is perhaps better mapped. As its surveyor, even if armed 
with the most powerful of telescopes, is still practically 
several hundred miles away, his map does not represent 
the smallest features ; but as all parts are equally accessible 
and as he has labored industriously these many years, there 
is no remaining space on which to write the legend “ unex¬ 
plored.” Upon his map are a score of great plains with dark 
floors, which he calls maria; there are a score of mountain 
chains; there are a few trough-like valleys remarkable for 
their straightness; there are many thousand circular valleys 
With raised rims, which it is convenient this evening to call 
craters,t although for the purposes of detailed description 
*A supplementary communication on the same subject was made to 
the Society at the meeting of January 7, 1893. The substance of that 
communication, as well as the results of later studies and experiments, 
are included in this publication. 
An outline of the discussion was read to the National Academy of 
Science in November, 1892, and was reported in abstract in the American 
Naturalist, vol. 26 (1892), pp. 1056-1057. A similar outline was presented 
to the New York Academy of Science in February, 1893, and is reported 
in abstract in vol. 12, pp. 93-95, of the Transactions. The same abstract 
appeared in Astronomy and Astro-Physics for March, 1893, No. 113, p. 286. 
f The word crater, derived from the Greek name of a kind of bowl, is 
used chiefly to designate the bowl-shaped cavities of volcanoes. In this 
paper, as in most selenographic writings, it designates a topographic form 
without implication as to the origin of the form. 
34-Bull Phil. Soc., Wash., Vol. 12. 
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