Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

The most curious feature on the moon's surface are the bands of lighter colour, which, radiating from certain of the volcanolike pits—those of lesser size and probably of latest origin—extend in some cases for five hundred miles or more across the surface. These light bands have never been adequately explained. It seems most likely that they are stains along the sides of cracks, such as are sometimes observed about volcanoes.

The eminent peculiarity of the moon is that it is destitute of any kind of gaseous or aqueous envelope. That there is no distinct atmosphere is clearly shown by the perfectly sharp and sudden way in which the light of a star disappears when it goes behind the moon and the clear lines of the edge of the satellite in a solar eclipse. The same evidence shows that there is no vapour of water; moreover, a careful search which the writer has made shows that the surface has none of those continuous down grades which mark the work of water flowing over the land. Nearly all of the surface consists of shallow or deep pits, such as could not have been formed by water action. We therefore have not only to conclude that the moon is waterless, but that it has been in this condition ever since the part that is turned toward us was shaped.

As the moon, except for the slight movement termed its "libration," always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold. There are reasons why this is improbable. In the first place, we see on the extreme border of the moon, when the libration turns one side the farthest around toward the earth, the edge of a number of the great walled pits such as are so plenty on the visible area; it is fair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side. Furthermore, there are certain bands of light which, though appearing on the visible side, evidently converge to some points on the other. It is reasonable to suppose that, as all other bands radiate from walled pits, these also start from such topographic features. In this way certain likenesses of the hidden area to that which is visible is established, thus making it probable that the whole surface of the satellite has the same character.

Clearly as the greater part of the moon is revealed to us—so clearly, indeed, that it is possible to map any elevation of its surface that attains the height of five hundred feet—the interpretation of its features in the light of geology is a matter of very great difficulty. The main points seem to be tolerably clear; they are as follows: The surface of the moon as we see it is that which was formed when that body, passing from the state of fluidity from heat, formed a solid crust. The pits which we observe on its surface are the depressions which were formed as the mass gradually ceased to boil. The later formed of these openings are the smaller, as would be the case in such a slowing down of a boiling process.

As the diameter of the moon is only about one fourth of that of the earth, its bulk is only about one sixteenth of that of its planet; consequently, it must have cooled to the point of solidification ages before the larger sphere attained that state. It is probable that the same changeless face that we see looked down for millions of years on an earth which was still a seething, fiery mass. In a word, all that vast history which is traceable in the rocks beneath our feet—which is in progress in the seas and lands and is to endure for an inconceivable time to come—has been denied our satellite, for the reason that it had no air with which to entrap the solar heat and no water to apply the solar energy to evolutionary processes. The heat which comes upon the moon as large a share for each equal area as it comes upon the earth flies at once away from the airless surface, at most giving it a temporary warmth, but instituting no geological work unless it be a little movement from the expansion and contraction of the rocks. During the ages in which the moon has remained thus lifeless the earth, owing to its air and water, has applied a vast amount of solar energy to geological work in the development and redevelopment of its geological features and to the processes of organic life. We thus see the fundamental importance of the volatile envelopes of our sphere, how absolutely they have determined its history.

It would be interesting to consider the causes which led to the absence of air and water on the moon, but this matter is one of the most debatable of all that relates to that sphere; we shall therefore have to content ourselves with the above brief statements as to the vast and far-acting effects which have arisen from the non-existence of those envelopes on our nearest neighbour of the heavens.

Methods in studying Geology.

So far as possible the preceding pages, by the method adopted in the presentation of facts, will serve to show the student the ways in which he may best undertake to trace the order of events exhibited in the phenomena of the earth. Following the plan pursued, we shall now consider certain special points which need to be noted by those who would adopt the methods of the geologist.

At the outset of his studies it may be well for the inquirer to note the fact that familiarity with the world about him leads the man in all cases to a certain neglect and contempt of all the familiar presentations of Nature. We inevitably forget that those points of light in the firmament are vast suns, and we overlook the fact that the soil beneath our feet is not mere dirt, but a marvellous structure, more complicated in its processes than the chemist's laboratory, from which the sustenance of our own and all other lives is drawn. We feel our own bodies as dear but commonplace possessions, though we should understand them as inheritances from the inconceivable past, which have come to us through tens of thousands of different species and hundreds of millions of individual ancestors. We must overlook these things in our common life. If we could take them into account, each soul would carry the universe as an intellectual burden.

It is, however, well from time to time to contemplate the truth, and to force ourselves to see that all this apparently simple and ordinary medley of the world about us is a part of a vast procession of events, coming forth from the darkness of the past and moving on beyond the light of the present day. Even in his professional work the naturalist of necessity falls into the commonplace way of regarding the facts with which he deals. If he be an astronomer, he catalogues the stars with little more sense of the immensities than the man who keeps a shop takes account of his wares. Nevertheless, the real profit of all learning is in the largeness of the understanding which it develops in man. The periods of growth in knowledge are those in which the mind, enriched by its store, enlarges its conception while it escapes from commonplace ways of thought. With this brief mention of what is by far the most important principle of guidance which the student can follow, we will turn to the questions of method that the student need follow in his ordinary work.