Let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth. According to this doctrine the earth was never in the form of a highly heated gas, nor was it ever necessarily hotter than now. Instead of beginning as a much larger body which has gradually diminished in size, the earth steadily grew, up to a certain stage, by ingathering of planetesimals. Increase in size caused the force of gravity to increase and this caused not only steady contraction of the earth’s matter, but also a development of greater internal heat. The earth has been getting smaller ever since the force of compression has predominated over the building-up process, because of the diminishing supply of planetesimals. Due to steadily increasing internal pressure and heat the various gases, including water vapor, have been driven out of the earth to form an atmosphere which has gradually become larger and denser. After sufficient accumulation of water vapor, condensation and rainfall took place; the waters of the earth began to gather to form the oceans; and the ordinary geologic processes of erosion and deposition of strata were initiated. According to this view stratified rocks could have been formed very early in the history of the earth, and in this connection it is interesting to note that the oldest known rocks are actually of sedimentary origin.
VERY ANCIENT EARTH HISTORY
(Archeozoic and Proterozoic Eras)
W
WE shall now consider the older rocks of the Earth, including those of Archeozoic, Proterozoic, and Paleozoic ages. What are the salient points in the very early history of the earth (not including the evolution of organisms) shown by these very ancient rocks? Beginning with the oldest known rocks, it will be our purpose to trace out the principal recorded events of earth history in the regular order of their occurrence. As in human history, so in earth history the recorded events of very early times are fewest and most difficult of all to understand. In spite of this difficulty it is best to begin with the oldest known rocks or, as Le Conte has said, “to follow the natural order of events. This has the great advantage of bringing out the philosophy of the history—the law of evolution.” Because of limitation of space we shall give special attention to the physical history of North America, but the general principles brought out apply almost equally well to the other continents.
The Archeozoic rocks contain the earliest known records of geological history, or, in other words, the oldest recorded ordinary geological processes such as weathering and erosion, deposition of strata, igneous activity, etc. Although we are here dealing with the most obscure records of any great rock system, partly because the rocks have been so profoundly altered (metamorphosed), and partly because of the absence of anything like definitely determinable fossils, it is, nevertheless, true that certain very important conclusions have been reached regarding this very ancient geological era.
Among the very oldest of all known rocks of North America are the Grenville strata, so named from a town in the St. Lawrence Valley. In fact, no rocks elsewhere in the world have been proved to be more ancient. The Grenville series consists of a great mass of sediments (strata)—original muds, sands, and limes—which were deposited layer upon layer under water ([Plate 12]). The widespread extent and character of the series in southeastern Canada and the Adirondacks, and more than likely far beyond these limits, make it certain that the Grenville strata were accumulated on the bottom of a relatively shallow sea very much as sediments are now piling up on shallow sea bottoms. Thus, the most ancient definitely known condition of the region where the Grenville strata are exposed was an expanse of the sea covering the whole area. Wherever, in other parts of the world, the Archeozoic rocks have been studied, stratified rocks also seem to be the very oldest which are recognizable, but up to the present time no such rocks have been proved to be any older than, or even as old as, the Grenville series.