This is a problem which has given rise to a great deal of discussion, but a solution appears to be near. Independently, in many countries, observers have reached the conclusion that these rocks are divisible into two great series, a schist series composed chiefly of highly metamorphosed sedimentary and volcanic rocks, and a gneissoid granite series, which is intrusive and later than the former.

Assuming that this conclusion is true, at least as a working hypothesis, it involves certain curious consequences. Surface lava flows and volcanic tuffs, and still more, sedimentary rocks, necessarily imply a solid floor upon which they were laid down, but of this floor not a trace has anywhere been found. The question immediately arises, what has become of it? No answer to this question can yet be given, but apparently the most likely suggestion is that the ascending floods of molten magma, which gave rise to the gneissoid granites, must have melted and assimilated it. If this were only a local phenomenon, there would be nothing very surprising about it, but it would seem to be true of the entire globe, and this is a startling conclusion. We are then to suppose that a solid crust, however formed, was for a very long time sufficiently rigid and stable to allow a great thickness of sedimentary and volcanic rocks to be accumulated upon it and then was ingulfed and destroyed by a universally ascending magma, though it is not necessary to suppose that this took place simultaneously over the whole earth, or even within a relatively short period of time; it may have required ages in the accomplishment.

Furthermore, it must not be forgotten that remnants of the floor may yet be discovered in little-known regions. If this complete and universal assimilation actually took place, it is an absolutely unique phenomenon in the recorded history of the earth, though something more or less similar may have happened many times before that record began.

Many other hypotheses have been propounded to account for the origin of the Archaean rocks, but as they are not supported by any strong evidence, it is not worth while to consider them here; several of them have been formally abandoned by their authors.

That the oldest known rocks were not the first to be formed is manifest from the derivative nature of many of them, for sediments necessarily imply some preexisting rock to furnish the materials, and volcanic outbursts involve a solid surface through which they break.

From the extreme degree of dynamic and thermal metamor-phism which the Archaean rocks have undergone, we should not expect to find any recognizable fossils in them. On the other hand, there are indirect evidences that life was already present on the earth at that period. The limestones, iron ores, and graphite found in these rocks appear to have been organically accumulated, but it is possible that they were chemically formed, and so the evidence, while probable, is not altogether conclusive.