BASIS OF PRE-CAMBRIAN CORRELATION 10g 
series were deposited on some floor, although this floor has remained 
undiscovered up to the present time. Either the Laurentian gneiss, 
or some part of it, represents the original floor, subsequently melted 
and intruded into the overlying sediments, or the original floor remains 
unrecognized among the enormous bodies of intrusive rocks which 
resemble it in character. 
Resting on this Laurentian complex, in the region of the Great 
Lakes, although penetrated by it, the lowest sedimentary series here 
recognized is the Keewatin series, a great body of rocks largely of 
pyroclastic origin, but in some districts containing great thicknesses 
of epiclastic material. 
It is not necessary here to make further reference to this great 
series which has been so well described by so many writers. In this 
region it is the oldest sedimentary rock recognizable as such. 
In the region of the St. Lawrence Valley this Keewatin is not seen, 
but there is a series of extraordinary thickness and enormous areal 
extent composed essentially of limestones, which rocks are practically 
absent in the Keewatin. Whether this series, known as the Grenville 
series, is the equivalent of the Keewatin is unknown as yet. If it be, 
the designation of the Keewatin by Van Hise as a series composed 
essentially of pyroclastic material to which stratigraphic methods 
cannot be applied and the assumption that such material characterized 
the earliest stratified deposits of the earth’s history, must be aban- 
doned, for the Grenville series is distinctly stratified and is one of 
the greatest limestones series in the earth’s crust. However that may 
be, these two series constitute the oldest sediments in the earth’s 
crust recognizable as such in their respective districts. Similar 
rocks apparently characterize extensive areas in the more northern 
and remote portions of Laurentia representing the oldest recognizable 
sediments in these districts. . 
At the close of this first period of long-continued sedimentation 
there came an epoch of diastrophism—a thrust exerted from a south- 
easterly direction against the ancient continent threw these series 
into a succession of great folds running approximately parallel to the 
present valley of the St. Lawrence. Enormous bodies of granitic 
magma rose in great bathyliths along the axes of the folds, disinte- 
grating, fraying out, metamorphosing and partially absorbing the 
