Page:The Moon (Pickering).djvu/27

Rh This low, irregular coast line is very different from that of the Pacific. The outline of the latter is smooth and nearly that of a great circle whose centre lies on the Tropic of Capricorn in longitude 153° W, It is bounded everywhere from Cape Horn to the East Indies by a continuous row of active or extinct volcanoes, while a few thousand miles north of the centre of this area rises perhaps the finest group of volcanic peaks on the whole globe. This dissimilarity of outline has never been explained. No similar configurations are found upon either the Moon, Mercury or Mars, the only other bodies in the universe whose permanent surface features are as yet open to our scrutiny.

The somewhat fanciful suggestion has been made that the great depression now occupied by the Pacific Ocean indicates the spot once filled by the Moon, and that the eastern and western continents were torn asunder at the time of that great convulsion, floating like two huge ice floes on the denser, partially metallic, fluid of the Earth's interior. Later, when the surface had sufficiently cooled, these huge depressions were filled with water, forming our existing oceans.

A body of the size of the Moon would equal in volume a section of the Earth's crust having an area equal to the terrestrial oceans and a uniform depth of thirty-five miles. Since the mean depth of the ocean bottom may be taken at about three miles below the continental surface, this would indicate that a solid crust thirty-five miles in thickness was floating to a depth of thirty-two miles on the Earth's liquid interior. This would give a not improbable value for the specific gravity of the liquid interior, nor is thirty-five miles an improbable value for the thickness of the terrestrial crust at that date. The Moon's specific gravity (3.5 times that of water) is also a probable value for the density of a layer of the Earth's crust of that thickness. Moreover, if the Earth had a solid crust at that time, such a catastrophe would be almost certain to leave some permanent scar upon it.

While we at present know of no reason for rejecting this curious supposition, we must at the same time recollect that we do not know that all of the materials which form the Moon were carried off at the same time, or from the same place; nor do we even know that any portion of the Earth's crust was solid at that time. The supposition must remain, therefore, a mere conjecture, incapable either of proof or denial