12 FRANK D. ADAMS 



The great expanse of Laurentia is underlain predominantly by 

 the rocks of the Laurentian system. These consist of gneisses in 

 infinite variety which in the majority of cases have the mineralogical 

 composition of granite, although some present foliated varieties of 

 rocks ranging from syenite to diorite. The foliation is in some cases 

 so faint that it can be detected only on large weathered surfaces, but 

 generally it is c^uite distinct or even striking. In addition to the 

 foliation the rock often displays a very distinct banding due to the 

 alternation of varieties of diverse character or composition. This 

 foliation is in many, and possibly in the majority of cases, a primary 

 structure and the darker bands very frequently represent included 

 masses of overlying rocks, softened and in some instances partially 

 digested. This foliation and banding was at one time regarded as a 

 partially obliterated bedding and considered to present indisputable 

 evidence that the rocks were of sedimentary origin. These gneissic 

 rocks are not all of the same age, for frequently one mass can be seen 

 to cut another. In addition to these gneissic granites, syenites, and 

 diorites, however, the Laurentian comprises other kinds of plutonic 

 rocks of very diverse character. Thus, from Minnesota to the shores 

 of Ungava Bay, intrusions of anorthosite are found. Several of these, 

 for the most part distributed along the margin of the protaxis in the 

 province of Quebec and in the Ungava peninsula, present areas of 

 from a few miles to 10,000 square miles in extent, and represent some 

 of the more recent pre-Cambrian plutonics, although they themselves 

 have been cut by still later granites. In fact, it is becoming more 

 and more evident with the progress of geological investigation that 

 the Laurentian is a vast complex of plutonic rocks of widely varying 

 types and differing greatly in age, although there is no evidence to 

 show that any of them were intruded later than the close of the Pro- 

 terozoic. Whether in this enormously extended complex, which we 

 term the Laurentian in the northern protaxis, there still survive any 

 primitive sediments or any portion of an original crust, through which 

 these great bodies of intrusive rocks forced themselves, is unknown. 

 None have as yet been distinguished with certainty, but if any do 

 exist they are probably similar in composition to these earliest intru- 

 sive rocks and might easily escape notice. 



It is certain, however, that the overlying Keewatin and Grenville 



