208 ELIOT BLACKWELDER 
ing from Labrador on the northeast (to the right) to southern Texas 
on the southwest (to the left). On the right, one sees that Labrador 
has been eroded from some time before the Cambrian down to the 
glacial period; and even then the deposition was accomplished by 
glaciers—a terrestrial agency. The unconformity there represents 
XAS CENTRAL LAKE 
GION INTERIOR REGION [LABRADOR 
Quatern ary 
Tertiary 
Cretaceous 
Jurassic 
Triassic 
Permian 
Pennsylvanian 
Mississippian 
Devonian 
Silurian 
Ordovician 
Cambrian 
Pre-Cambrian 
Fig. 4.—Diagram of an unconformity with lateral extensions and restrictions. 
The extent and duration of the principal periods and areas of sedimentation, with 
their corresponding rock systems, are shown in solid black. The white, on the other 
hand, denotes the time and extent of erosional conditions and corresponding uncon- 
formities. 
a vast and presumably uninterrupted lapse of time. When traced 
southwestward, however, this great denudation-interval branches 
into many minor intervals which are intercalated between periods 
of sedimentation—the times during which the so-called sedimentary 
record was produced. The minor unconformities dovetail in with 
the sedimentary series, especially around the borders of the continent. 
