108 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 quite 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 afew 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 
