William H. Pickering, who is, perhaps, more familiar with the
telescopic and photographic aspects of the moon than any other American
astronomer, has recorded numberless instances of change in minute
details of the lunar landscapes. He regards some of his observations
made at Arequipa as "pointing very strongly to the existence of
vegetation upon the surface of the moon in large quantities at the
present time." The mountain-ringed valley of Plato is one of the places
in the lunar world where the visible changes have been most frequently
observed, and more than one student of the moon has reached the
conclusion that something very like the appearances that vegetation
would produce is to be seen in that valley.
Professor Pickering has thoroughly discussed the observations relating
to a celebrated crater named Linne in the _Mare Serenitatis_, and after
reading his description of its changes of appearance one can hardly
reject his conclusion that Linne is an active volcanic vent, but
variable in its manifestations. This is only one of a number of similar
instances among the smaller craters of the moon. The giant ones are
evidently entirely extinct, but some of the minor vents give occasional
signs of activity. Nor should it be assumed that these relatively slight
manifestations of volcanic action are really insignificant. As Professor
Pickering shows, they may be regarded as comparable with the greatest
volcanic phenomena now witnessed on the earth, and, speaking again of
Plato, he says of its evidences of volcanic action:
"It is, I believe, more active than any area of similar size upon the
earth.
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