GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 395 



examine the rock surface beneath the general covering of 

 glacial debris and stratified sediments which partially fill 

 it. To do this, those areas in which rock in place forms the 

 surface require to be mapped and their elevations noted; the 

 records of wells and other excavations which pass through the 

 superficial deposits should also be obtained and the character of 

 the underlying rock ascertained, as far as is practicable. When 

 sufficient data of this nature shall have been recorded, a contour 

 map of the basin can be drawn that will reveal the shape of 

 the depression with which the student has to deal. The depth 

 of the present lakes plus an estimated thickness of clay and mo- 

 rainal material covering their bottoms, will probably furnish the 

 only means of sketching contours over the deeper portions of 

 the basin. Even an approximately accurate map of this char- 

 acter cannot be constructed for a long time to come, but every 

 advance towards it will serve to make the problems to be studied 

 more and more definite. 



Something of the form of the rock -basin is already known 

 and several deep channels in its borders, now filled with drift, 

 have been discovered. The courses of buried channels con- 

 necting the basins of some of the present lakes have also been 

 approximately determined. It is not necessary at this time to 

 refer specifically to the discoveries that have been made, but it 

 may be stated that enough is known to assure us that the basin is 

 a depression in solid rock, the bottom of which is below sea level. 



2. Origm of the basin. The rocks in which the Laurentian 

 basin is situated are, with the exception of the Lake Superior 

 region, nearly horizontal and belong almost wholly to the Paleo- 

 zoic. The basin is essentially a depression in undisturbed 

 strata, and all who have considered its origin seem agreed that it 

 has been formed by excavation. A vast mass of horizontal strata 

 has been removed, leaving an irregular rim of undisturbed rocks 

 on all sides. The form of the depression is now obscured by 

 drift; the deeper portions contain stratified sediments which have 

 been deposited within it and it has been warped somewhat bv 

 orographic movement. 



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