BASIS OF PRE-CAMBRIAN CORRELATION 107 
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 whole, might even serve as a basis of correlation over the whole 
vast area. Laurentia, however, has not as vet 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 afford a basis for pre-Cambrian 
correlation, making use of this principle of critical diastrophic 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 comparatively easy 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. 
