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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 palseozoic series rest unconformably on a horizontal 

 floor denuded on the schists continuously with the bevelled upper sur- 



■■:! ■' .' ■ • ' ' m 



Figure 14. 



The Algonkian "wedge" between the crystalline foundation and the palaeozoic series; 

 looking north across the canj-on from nearHance's on the Coconino river. Constructed 

 from rough sketch. 



face of the wedge of the Grand canyon series. 1 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 c, p. 553) and by Freeh (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 

 (-, p. 508) and doubtfully described as either an intrusion or a surface flow by 

 Freeh (p 177), 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. 



