BASIS OF PRE-CAMBRIAN CORRELATION 13 



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 Hmestones 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 bathyhths along the axes of the folds, disinte- 

 grating, fraying out, metamorphosing and partially absorbing the 



