Robinson — Tertiary Peneplain of the Plateau District. 121 



Virgin River cuts the Hurricane fault in Utah,* except that 

 the throw of the fault would be reversed. It seems probable 

 that the displacement has been caused by faulting rather than 

 by folding, and that there have been two distinct movements. 

 The first would account for the difference in the stratigraphic 

 horizons on which the lavas rest, respectively, in the Black 

 Hills and Black Mesa, while the second faulting displaced the 

 lavas and initiated conditions favorable to the development of 

 the present topography. 



The surface underlying the basalt in the Black Hills has not 

 been definitely described, but as the character of this surface 

 is known both to the east and west, it may be supposed that it 

 is about as thoroughly peneplained as is the corresponding sur- 

 face in the Black Mesa, hi xhe Bradshaw Mountainsf is 

 found the southwestern limit of the peneplain. The floor on 

 which the lavas rest, in the region east of the mountains 

 proper, is thus described : 



" The thickness of the wide eastern basalt flows is very vari- 

 able; the upper surface is relatively horizontal, but the bottom 

 fits the hollows in the underlying granite topography. Thus, 

 near Richinbar the granite reaches the level of the surface of 

 the mesa at several points, while near Bumblebee, 2 miles to 

 the west, the contact of the lava and granite lies 800 feet 

 lower." 



Elsewhere, if one may judge from the structure sections, the 

 planation was more thorough. This is shown on those sections 

 which cut Yavapai schist areas. On section C-C, for instance, 

 the present surface of the schist departs from a straight line 

 joining the Bigbug Mesa lava and that east of Cedar Canyon 

 on an average of about 300 feet, the maximum being 800 feet 

 at Cedar Canyon. These figures indicate an amount of erosion 

 that compares favorably with that in the neighboring plateau 

 district since the eruption of the older basalts and confirms the 

 impression that considerable areas about the Bradshaw Moun- 

 tains may formerly have been peneplained. 



The entire belt of country immediately south of the Mogol- 

 lon Mesa is, from the point of view of this article, an unknown 

 region. The only description suggesting planation is that of 

 Marvine,:}: of the region west of Fort Apache. It is as follows : 



" West of the post is a more open country, the floor of which 

 tends to be of the Red Wall limestone, though the preserva- 

 tive effect of the hard basalt has been to keep it covered with 

 mesa-like buttes and ridges of the banded red and yellow 

 Lower Aubrey beds. . . 



* Huntington and Goldthwait, op. cit. , p. 225 and fig. 5. 

 f Jagger and Palache, Bradshaw Mountains folio, Ariz., No. 126, U. S. 

 G. S., 1905. 



^Op. cit., p. 219 and pi. iv. 



