172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The strata of the Grand canyon series are seen in the form of a wedge 
with its apex pointing westward, as in Figure 14; its lower members 
rest unconformably on an inclined floor of denuded schists, while the 
basal members of the paleeozoic series rest unconformably on a horizontal 
floor denuded on the schists continuously with the bevelled upper sur- 
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Fictre 14. 
The Algonkian “wedge’’ between the crystalline foundation and the paleozoic series; 
looking north across the canyon from near Hance’s on the Coconino river. Constructed 
from rough sketch. 
face of the wedge of the Grand canyon series.‘ The skyline of the 
Kaibab is, as has been pointed out above, the edge of a stripped struc- 
1 The view of this structure given by Powell (a, p. 212, Figure 79) is somewhat 
misleading, for it represents the inclined members of the Grand canyon series with 
a horizontal base, as if they were examples of cross bedding on a gigantic scale. 
The correct relation is shown in figures by Walcott (b, p. 551; e, p. 507 — this 
from a drawing by Gilbert — and ¢, p. 553) and by Frech (p. 477). It may be noted 
that a sheet of basalt or diabase, which occurs near the base of the Unkar series 
and which has been regarded as contemporaneously interbedded by Walcott 
(e, p- 508) and doubtfully described as either an intrusion or a surface flow by 
Frech (p. 477), seemed to us to be an intrusive sheet or sill, because it appeared to 
step from one bedding plane to another by distances of a score of feet or more at 
one or two places; but this opinion is based only on field-glass observation at a 
distance of over a mile. 
