BASIS OF PRE-CAMBRIAN CORRELATION ii 



and back to the mouth of the Mackenzie. Beneath the Cretaceous 

 of western Canada, the margin of this element lies hidden. It ranges 

 past Lake Winnipeg toward the state of Wisconsin, and then follows 

 the shore of the Paleozoic mediterranean east to the Adirondacks and 

 St. Lawrence. 



Now it would seem, if we select a single positive element such as 

 Laurentia — remembering that the critical diastrophic periods will 

 be short and the intervening periods of deposition and accumulation 

 will be of long duration — that these epochs of diastrophism, with their 

 development of schistose structure in the moving masses and the 

 associated phenomena of igneous intrusion, might be employed as a 

 basis for the subdivision of Proterozoic time, and if the element moved 

 as a ^^■hole, might even serve as a basis of correlation over the whole 

 vast area. Laurentia, however, has not as yet been studied geologic- 

 ally except in a general way. Its detailed study will supply problems 

 for generations of geologists yet unborn. Its southern margin alone, 

 and that only in a few comparatively small areas, has been mapped in 

 detail, but nevertheless exploratory and reconnaissance work has been 

 carried out over almost the whole of the great expanse of this ancient 

 continent chiefly by the officers of the Geological Survey of Canada, 

 so that we have a good general knowledge of the main outlines, at 

 least, of its geological history. It is proposed here to present a general 

 statement of the results obtained, as they bear upon the history of 

 Laurentia in pre-Cambrian times and aftord a basis for pre-Cambrian 

 correlation, making use of this principle of critical diastropliic epochs 

 and drawing evidence from the area as a whole, rather than from a 

 few restricted areas on its southern border. 



This task is rendered comparativelv easv owing to the fact that a 

 critical digest of the mass of information concerning the pre-Cambrian 

 rocks of the great central and northern portions of Laurentia, which 

 is found disseminated through the reports and papers by the various 

 geologists who have worked in this great area, has recently been pre- 

 pared by Dr. George A. Young, of the Geological Survey of Canada, 

 who has himself traveled very extensively in this northern country. 

 I am indebted to Dr. Young for permission to make use of this unpub- 

 lished material, but the original papers have been consulted in the 

 case of all the more important occurrences. 



