Besides the mountains there exist on the Moon a number of plains analogous in some sense to the “steppes” of Asia and the “prairies” of North America. These were termed “seas” in the early days of the telescope, because it was assumed that as they were so large and so smooth they were vast tracts of water. This supposition has long ago been overthrown, but the names have been retained as a matter of convenience. Hence it comes about that in descriptions of the Moon one meets with such names as _Mare Imbrium_, the “Sea of Showers”; _Mare Serenitatis_, the “Sea of Serenity”; _Mare Tranquillitatis_, the “Sea of Tranquillity”; and so on. It seems probable that the so-called seas represent in nearly its original form what was once the original surface of the Moon before the mountains were formed. A confirmation of this idea is to be found in the fact that though these plains are fairly level surfaces compared with the masses of mountains which hedge them in on all sides, yet the plains themselves are dotted over with inequalities (small elevations and pits), which seem to suggest that some of them might eventually have developed into mountains if the further formation of mountains had not been arrested by the fiat of the Creator. Though hitherto we have been speaking of the mountains of the Moon under that generic title, it is necessary for the reader to understand that the Moon’s surface exhibits everywhere remarkable illustrations of those geological processes which we on the earth associate with the word “volcano.” There cannot be the least doubt that the existing surface of the Moon, as we see it, owes all its striking features to volcanic action, differing little from the volcanic action to which we are accustomed on the earth. That this theory is well founded may be very easily inferred by comparing the structural details of certain terrestrial volcanoes and their surroundings with a typical lunar mountain, or indeed, I might say, with any lunar mountain. This point was very well worked out some 40 years ago by Professor Piazzi Smyth, who placed on pictorial record his results of an examination and survey of the Peak of Teneriffe. Any person seeing side by side one of Smyth’s pictures of Teneriffe and a picture of any average lunar crater would find great difficulty if the pictures were not labelled in determining which was which.