Detailed studies of the Precambrian basement complex, initiated mainly for economic reasons, have now shown that the record is in part decipherable; that considered as a whole, it is indeed complex; and that it represents a large part of the history of the earth. However, the geological histories of many individual parts are no more complicated than those of areas of comparable size in younger mountain-built belts.

The processes affecting the outer crust in Precambrian time differed but little from those now active. Differences in proportions of rocks of different types that make up the record of the Precambrian, in contrast

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to those of later time, can be accounted for in part by gradual changes in the compositions of the atmosphere and oceans and in part by the depth of erosion.

Methods of study used in younger rock groups are all useful in the Precambrian, but extreme metamorphism over wide areas, more abundant igneous intrusive masses, and a dearth of fossils useful in correlation make the interpretation of the record less certain.

Methods of classification and naming recommended by the American Commission on Stratigraphic Nomenclature are now being adopted by Precambrian geologists, some of whom have assisted in their formulation. This should lead to clearer understanding and better communication.

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