LIMITATIONS OF PRE-CAMBRIAN NOMENCLATURE 331 
of batholithic intrusion had been recognized in most of the pre- 
Cambrian subprovinces of the St. Lawrence basin. There is a very 
apparent reason, however, why two and only two batholithic 
intrusions can be recognized in a single locality, namely, that if a 
third batholith were intruded in a district where two batholiths 
were already present the evidence of the former presence of one or 
other of the older batholiths would probably disappear.’ 
If it be assumed that batholithic intrusions represent the 
interior portions of mountain chains it is obvious that the prolonged 
erosion, which generally follows an orogenic uplift, must inevitably 
result in the stripping off of the roof rocks from the underlying 
massif and the replacement of surface rocks by plutonic types in 
the district where the uplift has occurred; also that successive 
crustal movements of the orogenic type in the same or adjoining 
localities must eventually bring about the disappearance of all 
trace of rocks originally present in such zones of disturbance. It is 
probable that within the base-leveled pre-Cambrian complex 
which underlies the larger part of the Canadian shield evidence of 
the presence of more than two separate periods of batholithic 
intrusion would not generally survive in a single locality. I, 
however, the succession of formations can be determined over an 
extended area, as where less metamorphosed late pre-Cambrian 
sediments occur, the number of batholithic intrusions which can 
be recognized might be increased. Thus, as a result of the more 
extended areal geological studies of recent years, evidence is accu- 
mulating that at least three definite periods of batholithic invasion 
are represented in several of the pre-Cambrian subprovinces of the 
St. Lawrence basin. 
The folded and metamorphosed pre-Cambrian rocks occurring 
along the southern margin of the Canadian shield have in the main 
a northeasterly structural trend; likewise the granitic batholiths, 
so far as their areal distribution has been determined, are distributed 
in northeasterly trending zones; thus the region (approximately 
1,000 miles in length) extending from the Adirondacks to the Lake 
of the Woods, to which Lawson’s hypothesis has been applied, lies 
almost transverse to the regional trend of pre-Cambrian folding, 
tA.C. Lane, Am. Jour. Sct., XLIII (1917), 42. 
