At the outset of our historical studies it is essential to understand clearly just what is meant by the term distribution of a given formation. It means: (i) that the given rock is at the surface over a certain area, disregarding the covering of soil, drift, or other loose materials; (2) that the concealed extension of the formation beneath newer rocks may be confidently inferred from surface observations. So far as the Archaean rocks are concerned, their surface distribution can at present be stated only with much reserve, for they often grade into crystalline schists of demonstrably later date, and much that once was referred to the Archaean is now known to be far more recent. Accurately to determine the distribution of the basal complex will require the most extensive, minute, and laborious.

Fig. 258. - Map of known pre-Cambrian surface exposures in North America.

The black areas are outcrops investigation. The northern part of North America, from the Arctic Ocean to the Great Lakes, probably including Greenland, is made up of an immense area of schistose rocks, estimated at more than 2,000,000 square miles in extent. Over this vast region occur numerous areas of Archaean rocks, but it is not yet possible to say how much of it belongs in that group and how much is newer.

Beside this principal region are several other minor ones. A narrow band of schistose rocks extends, with some interruptions, from Newfoundland to Alabama, with shorter parallel belts in eastern Canada and New England. Another great axis is on the site of the Rocky Mountain chain, with several shorter and generally parallel belts from Mexico to Alaska. Isolated areas occur in Missouri, central Texas, New Mexico, and Arizona. In all of these regions are found rocks like the typical Archaean, which stand in the same relation to the newer groups, but how much should be referred to the oldest series is still a question.

In the other continents occur great areas of very ancient gneisses and crystalline schists, but even less, than in North America has the distinction been made between the fundamental complex and newer groups. In the following statements no attempt is made to determine how much of the areas mentioned is properly Archaean.

In Europe the principal area lies to the north, covering parts of Ireland and the Highlands of Scotland, with which was probably once connected the great continuous mass of Scandinavia, Finland, and Lapland. Considerable areas also occur in central and southern Europe, as the central plateau of France, parts of Germany and Bohemia, and long, narrow belts in the Pyrenees, Alps, and Balkans. In Asia these ancient crystalline rocks are found in the great mountain ranges, such as the Himalayas, Altai, etc. They make up a large part of the Indian peninsula, and are extensively displayed in China, Japan, and the islands of the Malay Archipelago. The vast central plateau which occupies so much of Africa is principally composed of these rocks, which are also largely exposed in Australia. In South America similar rocks appear in the highlands of Brazil and in the Andes.

It is estimated that the Archaean rocks form somewhat more than one-fifth of the land-surface of the earth, and there is reason to believe that they are actually universal, and that a boring made at any point, if sufficiently deep, would encounter them. They are found at the bottom of many deep canons, and borings frequently penetrate them at points where there are no surface indications of their presence. If these rocks are really distributed over the entire globe, they are the only formation of which this is true.