Robinson — Tertiary Peneplain, of the Plateau District. 117 



small cones. It is necessary, therefore, to form an idea of the 

 surface underlying the lavas from the outcropping strata at the 

 edges of the mesa. 



On the western side these strata belong predominantly to the 

 Upper Aubrey limestone and sandstone formations. On the 

 eastern side, however, the overlying red beds of the Moencopie 

 and Shinarump formations outcrop beneath the lava cap. The 

 strata possess a slight dip to the northeast, and it is evident 

 that the lava rests upon, the eroded edges of the beds. In 

 view of the peneplained condition of the contact between the 

 lava and underlying sedimentaries at the north end of the mesa, 

 Anderson Mesa section, figure 2, and in the region farther 

 north, it is regarded as practically certain that the peneplain 

 exists under the older lava throughout the entire extent of the 

 mesa, although the surface may be slightly more irregular in 

 the vicinity of Beaver Creek. 



The presence of isolated, lava-capped buttes beyond the 

 eastern edge of the mesa makes it certain that the peneplain 

 once had a greater extension in that .direction, and it is con- 

 sidered as highly probable that it formerly extended, as did 

 the peneplain in the San Francisco Mountain region, somewhat 

 to the north of the present position of the Little Colorado 

 River. That it originally had a greater extension westward 

 into the present mountain district of Arizona, a point that 

 will be referred to later, would appear equally certain, since 

 the western boundary is now marked by a cliff due to erosion. 



Mogollon Mesa. — This mesa is a continuation of the Black 

 Mesa and extends in a direction a little south of east for a dis- 

 tance of 100 miles from Clear Creek Canyon to the White 

 Mountains. On the north the boundary of the mesa is not 

 always distinctly marked. The southern edge, on the contrary, 

 is sharply limited by the Aubrey Cliff, which, where it over- 

 looks the Tonto Basin, closely duplicates the upper 2,000 feet 

 of the walls of the Grand Canyon of the Colorado. 



Marvine traversed the mesa in journeying from the Little 

 Colorado River to Fort Apache, and his report, with cross- 

 section, is given in Volume III of the Survey West of the 

 100th Meridian.* He states that the lava capping the mesa is 

 " identical with that upon the Black Mesa." His section shows 

 that the lava on the north side of the mesa is underlaid either 

 by the Upper Aubrey sandstone or by the uppermost beds of 

 the Lower Aubrey sandstone. On the southern side the strata 

 are "the Lower Aubrey sandstonesf of a horizon below the 

 beds that occur on the north side of the mesa. 



* Pages 215-218 and pi. iv. Note that the south end of cross-section is 

 incorrect. See foot-note on page 217. 



f Reagan, Geology of the Fort Apache Region, Am. GeoL, Nov., 1903. 



