The region lying along the southern border of the Canadian Shield, the St. Lawrence basin, to which by far the greater part of the field work so far performed in this vast area has been confined, is divided into four geographically separate sub-provinces, in each of which the succession of formations is so different that few positive correlations of either formations or unconformities in different sub-provinces can be made. It is probable, however, that all geologists familiar with this region are agreed that there is a palaeoplain at the base of the Huronian that originally at least extended over the southern part of the Canadian Shield. If the existence of this palaeoplain be assumed as established then there are two major problems in Pre-Cambrian classification in this region that remain unsolved: (1) What is the position of the pre-Huronian palaeoplain in the succession of formations present in the sub-province north-west of Lake Superior and in the Grenville sub-province? and (2) is there a second erosion plain within the pre-Huronian complex, and, if so, can it be used satisfactorily for the classification of the pre-Huronian into two major divisions equivalent in rank to the Huronian?

The purpose of this paper has been to point out (1) that there is no present agreement regarding the solution of either of these problems; (2) that the existence of a widespread unconformity within the pre-Huronian complex of the St. Lawrence province is, as yet, only a hypothesis, and (3) that there are only two logical courses to be followed in the classification of the Pre-Cambrian formations of the Canadian Shield, either to adopt the dual classification or to use it tentatively in the manner indicated in the accompanying table, until the existence of the second widespread unconformity is established or disproved. It may be suggested, however, that since considerable difficulty is being encountered in applying the dual classification even within the St. Lawrence province, it may be more practical to adopt the dual classification for the major divisions of the Pre-Cambrian notwithstanding the possible presence of an unconformity within the pre-Huronian complex.

In conclusion the writer wishes to express his indebtedness to his colleagues of the Geological Survey of Canada, W. H. Collins, F. J. Alcock, T. L. Tanton, and J. F. Wright, for criticism of the columns in the table of formations having reference to the different sub-provinces with which they are especially familiar.