PROBABILITY OF METEORITES HAVING FALLEN 129 
eravitative attraction on the moon being only one-sixth what it is on 
earth, would allow very steep slopes at the front of even fluid lava 
flows. The origin of this lava is still hypothetical, but it is to be noticed 
that none of the volcanoes of the moon give forth freely flowing 
streams of lava, nor do any of the numerous fissures or faults on the 
lunar surface, some of which evidently penetrate deeply, distinctly 
give rise to lava flows; generally it is established that all the volcano- 
like openings appear always to have retained their lavas within or 
near their walls, or, in other words, there was no tendency for lava to 
pass up to the surface in large quantities. 
There is no evidence in any of the maria that the lava came up 
from a central pipe or from an elongate fissure; the general form 
of the seas is rounded or oval, and it would seem to indicate that if 
the fluid came from within, the lava should have emerged as from a 
terrestial volcano pipe, for if it came from fissures these should have 
been of elongate shape. But if the lava came either from fissured 
or from pipe-like openings there should be a grade to the flow extend- 
ing from the center of the field to its margin; owing to the slight value 
of gravitation this grade should be steep. ‘There, however, is no trace 
of such a slope; on the contrary, the curve of the margin of illumina- 
tion shows the surfaces of the maria are essentially horizontal. 
The hypothesis which fulfils most of the conditions of the case 
with respect to the origin of the lava of these maria in the moon is 
that great meteors fell upon the moon and by their impact produced 
sufficient heat not only to melt up their own substance but a good 
deal of that comprising the adjacent lunar surface. Even beyond the 
seat of impact, the shearing strains would probably be sufficient 
to convert much of the material of the surface into a fluid state, with 
the result that a mass of lava of a very high temperature, equal at 
least to the bulk of the invading body and probably several times as 
great, would be sent radially from the point where the impact took 
place. The evidence of melting effected by the material which forms 
the plains of the maria is considerable at several points, notably in 
the case of the craters on the margins of the seas. It seems quite 
certain that the walls of these craters next the sea have been in some 
manner effaced by contact with the material which came against it; 
in the case of the crater Flamstead in the Oceanus Procellarum, the 
