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

by Nathaniel Southgate Shaler

Although the aspect of the record of life since the beginning of the Cambrian time indicates a period of at least a hundred million years, it must not be supposed that this is the limit of the time required for the development of the organic series. All the important types of animals were already in existence in that ancient period with the exception of the vertebrates, the remains of which have apparently now been traced down to near the Cambrian level. In other words, at the stage where we first find evidence of living beings the series to which they belong had already climbed very far above the level of lifeless matter. Few naturalists will question the statement that half the work of organic advance had been accomplished at the beginning of the Cambrian rocks. The writer is of the opinion that the development which took place before that age must have required a much longer period than has elapsed from that epoch to the present day. We thus come to the conclusion that the measurement of duration afforded by organic life indicates a yet more lengthened claim of events, and demands more time than appears to be required for the formation of the stratified rocks.

The index of duration afforded by the organic series is probably more trustworthy than that which is found in the sedimentary strata, and this for the reason that the records of those strata have been subjected to numerous and immeasurable breaks, while the development of organic life has of necessity been perfectly continuous. The one record can at any point be broken without interrupting the sequences; the other does not admit of any breaches in the continuity.

The Moon.

Set over against the earth—related to, yet contrasted with it in many ways—the moon offers a most profitable object to the student of geology. He should often turn to it for those lessons which will be briefly noted.

In the beginning of their mutual history the materials of earth and moon doubtless formed one vaporous body which had been parted from the concentrating mass of the sun in the manner noted in the sketch of the history of the solar system. After the earth-moon body had gathered into a nebulous sphere, it is most likely that a ring resembling that still existing about Saturn was formed about the earth, which in time consolidated into the satellite. Thenceforth the two bodies were parted, except for the gravitative attraction which impelled them to revolve about their common centre of gravity, and except for the light and heat they might exchange with one another.

The first stages after the parting of the spheres of earth and moon appear to have been essentially the same in each body. Concentrating upon their centres, they became in time fluid by heat; further on, they entered the rigid state—in a word, they froze—at least in their outer parts. At this point in their existence their histories utterly diverge; or rather, we may say, the development of the earth continued in a vast unfolding, while that of the moon appears to have been absolutely arrested in ways which we will now describe.

With the naked eye we see on the moon a considerable variation in the light of different parts of its surface; we discern that the darker patches appear to be rudely circular, and that they run together on their margins. Seeing this little, the ancients fancied that our satellite had seas and lands like the earth. The first telescopes did not dispel their fancies; even down to the early part of this century there were astronomers who believed the moon to be habitable; indeed, they thought to find evidence that it was the dwelling place of intelligent beings who built cities, and who tried to signal their intellectual kindred of this planet. When, however, strong glasses were applied to the exploration, these pleasing fancies were rudely dispelled.

Seen with a telescope of the better sort, the moon reveals itself to be in large part made up of circular depressions, each surrounded by a ringlike wall, with nearly level but rough places between. The largest of these walled areas is some four hundred miles in diameter; thence they grade down to the smallest pits which the glass can disclose, which are probably not over as many feet across. The writer, from a careful study of these pits, has come to the conclusion that the wider are the older and the smaller the last formed. The rude elevations about these pits—some of which rise to the height of ten thousand feet or more—constitute the principal topographic reliefs of the lunar surface. Besides the pits above mentioned, there are numerous fractures in the surface of the plains and ringlike ridges; on the most of these the walls have separated, forming trenches not unlike what we find in the case of some terrestrial breaks such as have been noted about volcanoes and elsewhere. It may be that the so-called canals of Mars are of the same nature.

Fig. 23.—Lunar mountains near the Gulf of Iris.