322 The American Geologist. May, 1891 
to show in a former paper.* In that complex there are recogniz- 
able, in the region which I have examined, at least two great 
groups of stratified rocks, the Coutchiching and Keewatin, with 
a probable unconformity between them. This unconformity is in- 
ferred not from incongruity or discordance of structural planes 
but from the sharp contrast in the lithology of the respective for- 
mations as indicating a change in the condition of rock forma- 
tion, and from the presence of conglomerates near the base of the 
Keewatin as indicating a period of erosion. These two groups of 
strata have been folded and welded together by the same crust- 
crumpling forces and both bear identically the same relations to 
the great batholites of Laurentian gneiss and granite. That relation 
as I have elsewhere attempted to make clear is one which has 
arisen from the irruption through crustal rocks of a sub-crustal 
granitic magma. At the time when this irruption transpired the 
Coutchiching and Keewatin rocks co-existed forming the lower 
part of the crust, and the}' were together pierced and invaded by 
the common sub-crustal magma. The evidence which establishes 
this proposition is explicit and has been set forth in detail. It is 
this great and incontrovertible fact of the simultaneous invasion 
of both Coutchiching and Keewatin by the magma now recogniz- 
able as Laurentian foliated granite, which knits the complex to- 
gether and gives it an individuality and totality unique in struc- 
tural geology. Briefly then we have these considerations be- 
fore us : 
1. In their relations to the complex as a whole and to the 
Laurentian granites and gneisses which bind the complex together 
as a matrix, the Coutchiching and Keewatin are entirely similar. 
2. Both were firm brittle rock formations at the time when the 
Laurentian batholites were undifferentiated molten magma, hence 
by the criterion whereb}- the age of rocks in a geological sense is 
usually determined both are of younger age than the Laurentian 
which invaded them as irruptive masses. 
3. The whole complex, constituted as above sketched, ante- 
dates the great pre-palaeozoic hiatus or erosion interval which is 
probably the greatest in American geology. 
4. The above three statements are arrived at practically inde- 
pendently of any considerations as to the lithology or original 
character of the strata of either Coutchiching or Keewatin. 
*Xote on the pre-palseozoic surface of the Archaean terranes of Canada. 
Bull. Geol. Soc. Am. Vol. I. pp. 163-174. 
