This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
It is unfortunate that an account of historical geology should necessarily begin with the most difficult and obscure part of the whole subject, but the treatment must be in accordance with the chronological order, and the oldest rocks are the least intelligible. The ordinary criteria of the historical method, namely, the strati-graphical succession and the comparison of fossils, fail us here almost entirely, and the only way of correlating the rocks of different regions and continents is by means of the characters of the rocks themselves. In the present state of knowledge, "lithologi-cal similarity" is not a safe guide. So many metamorphic rocks, once referred to the Archaean, have proved to be of much later date, that some cautious geologists, who have no confidence in "lithological similarities," prefer not to use the term Archcean at all, but to employ local terms for the oldest crystalline rocks exposed in a given district.
The Archaean includes the most ancient rocks, often spoken of as the "basement, or basal complex." Its antiquity is best assured in regions where it is separated by thick series of sedimentary or metamorphic rocks from the Lower Cambrian, which can be certainly identified by its fossils. The character and relations of the pre-Cambrian rocks differ so much in different areas that it will be best to describe them in two or three typical regions. In the Canadian provinces of Quebec and eastern Ontario and in the Adirondack Mountains, the oldest rocks are a series of intensely metamorphosed sediments, including great bodies of limestones, quartzites, schists, etc. In the Adirondacks, especially, this series is invaded by enormous bodies of intrusives, which preceded and were involved in a great period of metamorphism. In eastern Ontario the thickness of the metamorphosed sedimentaries is exceedingly great. "Along its whole northern border this sedimentary series is torn to pieces by an enormous volume of gneissic granite of igneous origin which rises from beneath it, and which along its margin also wells up through it in the form of great intrusive bathyliths." Though underlying the metamorphosed sediments of the Grenville series, the gneissic granite, or Laurentian is the younger, as the contact is an intrusive one.
In the region around Lake Superior the pre-Cambrian rocks are displayed in enormous thickness and are divided into groups by four great unconformities. Of these the most ancient is the Kee-watin, intensely metamorphosed rocks derived from the transformation of lava flows, tuffs, and other volcanic rocks, with some of sedimentary origin, and forming a great series of schists, which are underlaid and penetrated by the Laurentian gneissic granites. It immediately suggests itself that the Grenville series of the east is the equivalent of the Lake Superior Keewatin, but the committee of the Canadian and United States Geological Surveys, which has investigated these problems, report that they consider it "inadvisable in the present state of our knowledge to attempt any correlation of the Grenville series with the Huronian or Keewatin." The classification proposed by the committee is contained in the subjoined table, though emphasis must again be laid upon the fact that no equivalence between the Grenville and the Keewatin is asserted.
They may be separated by vast periods of time, and yet both must be older than the intrusion of the Laurentian granites.
Lake Superior Region
N.B. - In the classification adopted in this book the Archaean comprises the Keewatin and Laurentian and probably also the Grenville series.
The Archaean, then, is composed of completely crystalline rocks of various types. Massive rocks, such as granite and basic erup-tives, and foliated rocks, like gneissoid granite, gneiss, many varieties of schists, are intermingled in the most intricate way, a characteristic well expressed in the oft-used phrase of the basal or fundamental complex for the Archaean. The component mineral particles show plainly the intense dynamic metamor-phism to which they have been subjected, in their extremely complex arrangement and in their laminated and crushed condition. The rocks thus referred to the Archaean are not necessarily all of the same age, but they are all of vast antiquity and older than any other known series. They are of very great but unknown thickness, for the bottom of them is nowhere to be seen, and even when thrown up into mountain ranges, erosion has in no case cut so deeply into these rocks as to expose anything different below them.
The reason for uniting these rocks into one group is not merely their likeness in composition, which is not a sufficient criterion, but because of their unique and uniformly complex structure, their resemblance to one another and difference from any other group of rocks, and their invariably fundamental position.