l6 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



the maria with the higher parts of the moon throughout the several thousand miles 

 of their extent will . probably come to the conclusion that they were formed by 

 the once fluid matter of the sea inundating firm land. Assuming, as I shall do, 

 that these maria are made up of vast bodies of lava, which came upon the surface 

 after the greater vulcanoids were made and, as we shall hereafter see, after some 

 of the radiating light streaks were formed, how shall we account for the produc- 

 tion of such bodies of igneous material ? The quantity of this matter was 

 evidently very great and in each of the seas it seems to have appeared all at 

 once, there being no mark of successive flows such as compose the extensive 

 lava fields of the earth. So far I have not been able clearly to trace any signs 

 of contact or over-lapping of the lava of the several maria. The search is, how- 

 ever, difficult ; no more has been ascertained than that the material must have 

 been extremely fluid, far beyond what is seen in ordinary terrestrial flows. This 

 is shown by the fact that although gravitative attraction is only one-sixth what 

 it is on the earth, there is no steep face at the front of the fields, such as oc- 

 curs from cooling of an ordinary stream of lava. 



As for the origin of the lava of the maria there are few facts on which to 

 base an hypothesis. What have been gathered may be briefly set forth. First, 

 it is to be noted that none of the vulcanoids of the moon give forth freely flow- 

 ing lava streams ; it is, indeed, doubtful if any true lava flows have come from 

 them. The features which suggest such streams are rare and rather inconclu- 

 sive ; they justify the statement that even the greatest, in general the earliest of 

 the craters, and therefore those which should have had the largest amount of 

 molten rock beneath them, show little or no signs of a tendency to extrude free 

 flowing lava at the time when they were formed. Nor do any of the numerous 

 fissures or faults of the lunar surface, some of which evidently penetrate deeply, 

 distinctly give rise to lava flows. And we shall see when we come to consider 

 the conditions of these volcano-like openings they appear always to have retained 

 their lavas within or near their vents. Clearly these vulcanoid openings do not 

 indicate any tendency of lava to pass up to the surface in large quantities. 



It is an important point that there is no evidence in any of the maria that 

 the lava comes from a central pipe or from an elongate fissure ; their general form 

 would seem to indicate that if the fluid came from within it should have emerged 

 as from a terrestrial volcanic pipe, for if it came from fissures these should have 

 been of elongate shape. But if it came either from fissured or from pipe-like 

 openings there should be a grade to the flow extending from the center of the 

 field to its margin ; owing to the slight value of gravitation this grade should be 

 steep. There seems to be no trace of such a slope ; on the contrary, the curve of 

 the terminator or margin of the illumination shows that they are essentially 

 horizontal. It is difficult to believe that lava flowing from an opening for hun- 

 dreds of miles could have this absence of slope. When it flows from a terrestrial 

 crater the course is always short and very steep. 



In view of all the facts, I am disposed to hold with Gilbert and other 

 inquirers that the maria are the result of large masses falling upon the surface of 



