PRECAMBRIAN STRATIGRAPHY 
55 
of these regions are the strata, altered scarcely more than it is 
customary to encounter among the Paleozoics. Since the beds 
are fossiliferous to depths of more than two miles beneath 
the typically earliest Cambric, or Olenellus, zone extensive 
faunas may be expected eventually to be disclosed. 
The fact that as yet no long sequences of faunas are deter- 
mined whereby the rock-sections of the various localities widely 
separated geographically may be analyzed, compared and 
grouped into units having definite taxonomic values, as is the 
common practice among the Paleozoics does not militate against 
the utilization to their fullest extent of certain physical fea- 
tures which have equal if not superior importance as strati- 
graphic and correlative criteria. These are the diastrophic 
features which find their most conspicuous expression in un- 
conformities. Their lateral extent, taxonomic rank and strati- 
graphic value are best indicated in diagram. These broader 
stratigraphic aspects of the American Pre-Cambrian complex 
along a given cross section, as the boundary line between Can- 
ada and the United States, are represented on the accompany- 
ing chart (Plate II). Both the relative amounts of sedimentation 
and the magnitude of the important stratigraphic hiatuses also 
are indicated. The latter mainly represent more or less great 
erosional intervals — times of notable emergences of the conti- 
nental tract. The enormous extent of denudation to which they 
point is no less astounding than the prodigious volume of the 
sedimentation in comparison with which some of the most fa- 
miliar Paleozoic sections sink into insignificance. 
Considered in their larger, or continental, relations the mag- 
nitude of sedimentation and the extent of removal give intrinsic 
suggestion of the taxonomic ranks to which many of the ter- 
ranes already recognized should be assigned. Conspicuous fea- 
tures also fo be especially noted are the almost uninterrupted 
degradation in the east ; the almost continuous sedimentation 
in the west; and the sweeping oscillatory movement of the an- 
cient strand-line in the continental interior. Particularly note- 
worthy also are the three major breaks in deposition marked 
by unconformities of continent-wide extent. The minor uncon- 
formities are likewise significant. Both larger and smaller 
unconformities are direct expressions of notable diastrophic 
movements. They are the basal horizons of new cycles of sedi- 
