34 



It will be noticed that here the term Laurentian is 

 used in a somewhat different sense from that in which it 

 was employed by Logan. In Logan's original classification 

 of the Laurentian this term — apart from the Upper 

 Laurentian, which was proved to be composed essentially 

 of anorthosite intrusions — included two series differing in 

 character, namely, the Lower Orthoclase ( Fundamental') 

 gneiss and the Grenville series. Now that investigations 

 have shown that these two series differ in origin (one 

 being essentially a great development of very ancient 

 sediments, and the other consisting of great bodies of 

 igneous rock underlying and intruded through them), it 

 becomes necessary to separate these two developments in 

 drawing up a scheme of classification. As the lower 

 gneisses, forming what has been termed the ' Fundamental 

 gneiss,' have an enormously greater areal development 

 than the overlying sedimentary series, constituting as they 

 do a very large part of the whole Northern Protaxis, and 

 forming the basis upon which the Grenville series rests, it 

 has been proposed that the term Laurentian be restricted 

 to this great development of igneous gneisses [5, p. 89 and 

 6, p. 191. 



The Grenville series is thus separated from the Lau- 

 rentian system, and the name is employed to designate 

 the sedimentary series which overlies the lower gneisses 

 and granites. The name Laurentian will, in addition to 

 its geological use, continue to have a geographical or 

 physiographical significance, as, for instance, in the term 

 Laurentian Protaxis, which latter forms so striking a 

 feature of the continent of North America, and is under- 

 lain chiefly by the gneisses of the Laurentian system. 



In its petrographical character and in the display of 

 the products of metamorphism which it presents, this 

 great area on the southern border of the Canadian protaxis 

 resembles in many respects certain classic localities of the 

 ' Grundgebirge ' on the continent of Europe [10], but in 

 none of them, with the possible exception of the Scandi- 

 navian peninsula, can the successive stages of metamor- 

 phism be so clearly traced, or its final products be studied 

 in such enormous development. The area is very instruc- 

 tive, as presenting a section of the appareils granitiques, 

 the 'roots of the mountains,' laid bare for study by the 

 processes of denundation. 



The Laurentian protaxis from early times has been 

 relatively an area of progressive uplift; while that of the 



