1892.] Proceedings of Scientific Societies. 1057 
earth. The plains lie lower than other portions of the surface, and 
are distinguished by their darker color. By those who have mapped 
the surface of the moon they are called seas, but the word is used in a 
figurative sense, for it is well understood that there is no water on the 
moon. The mountains are usually in the form of rings, each ring 
inclosing a hollow, and to this form the name crater is given. They 
are scattered over the surface of the plains, and on the uplands they 
are thickly set, overlapping one another in every variety of relation. 
They are of all sizes, from the smallest that the telescope can discern 
to a diameter of several hundred miles. Those of medium and larger 
size are usually characterized by a smooth circular plain in the inte- 
rior and a hill or group of hills rising in the center of the plain. 
They differ from the craters of the earth in various ways, especially in 
the fact that their bottoms are below the level of the surrounding 
country, and in the fact that the central hill bears no crater on its 
summit. . 
“The origin of these craters has been the subject of many theories. 
Despite their marked peculiarities of form, they have more commonly 
been ascribed to volcanic action; but they have also been referred to 
the bursting of gigantic bubbles, to the evaporation of water and its 
accumulation about the point of evaporation as ice, and to the impact 
of bodies from without. Personally, I favor the last mentioned expla- 
nation, but I differ from other writers in respect to the origin of the 
colliding bodies, It has been previously surmised that these might be 
rocks hurled from terrestrial volcanoes; that they might be meteors 
from the recesses of space, such as are continually burned in the upper 
layers of our atmosphere, giving rise to shooting stars, and that they 
might be aggregates of such meteors constituting balls of cosmic dust. 
Now my idea of their origin is based upon the phenomena of the 
planet Saturn and its ring. About that planet is a dise-like ring which 
astronomers believe to be constituted of an indefinitely large number 
of very small bodies revolving about the planet in parallel orbits—a 
symmetrically shaped form of small satellites. Assume that a similar 
ring of minute satellites once encircled the earth, and that these grad- 
ually became aggregated into a smaller number of larger satellites, 
and eventually into a single satellite—the moon. The craters mark 
the spots where the last of the small bodies collided with the surface 
when they finally lost their independence and joined the larger body. 
In Prof. G. H. Williams’ paper on The Volcanic Rocks of South 
Mountain in Pennsylvania and Maryland, he announced that during 
the past summer he had been able to clearly identify over 175 square 
