AO BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
here the Shinarump sandstone forms a strong bench, back from which 
the huge upper Trias cliffs, Plates 6 A, 7 A, have retreated from two to 
four miles, in consequence of active sapping by the underlying weak lower 
Trias clays. The general profile is shown in ABCDE, Figure 14. In 
following up the North fork, the valley or canyon becomes narrower 
until it is hardly more than a cleft : this is the remarkable Mukuntuweap 
canyon of which Gilbert gave an account thirty years ago (p. 79) and 
of which we saw the entrance last summer ; our own failure to enter it 
being due only to that oft-prevailing difficulty in geological excursions, 
a lack of time. The cleft has been given fame from the reproduction of 
Gilbert’s figure of it-on the back cover of Leconte’s Elements of Geology, 
where it has often been mistaken for a section of the Colorado canyon. 
The open lower canyon of the Virgin here narrows to an enclosed cleft 
Ficure 14. 
Generalized cross-section of the canyon of Virgin river. 
because the gentle northward dip of the strata combines with the north- 
ward ascent of the river bed to raise the stream from a mid-Permian to 
a mid-Trias horizon ; here it finds only the heavy and resistant sand- 
stone cliff-maker, B, almost free from partings on stratification surfaces, 
but well provided with vertical joints; as a result, the rocks at the 
canyon bottom exert no sapping action on those higher up the walls. 
The cross-section of the canyon at any intermediate point between 
Rockville and the cleft may be represented by imagining a vertical line 
at an appropriate position, as F, on the profile ABCDE, and reproducing 
the upper part of the profile, ABCF, on the right of the vertical, as 
A/BIC'F. ' 
It is only at higher levels than those exposed in the deeper part of 
the Virgin canyon that traces are found of an older topography, formed 
in an earlier cycle of erosion than that in which the canyon as a whole 
