92 WJMCGEE SHEETFLOOD EROSION. 



smooth that, except where outlying granite buttes rise from its expanse, 

 it may be traversed by wagons in any direction ; yet for 5 miles from the 

 mountain base on northeast, north, west, south, and southeast the wheels 

 grind over granite half the time, while on the east the alluvial veneer 

 appears barely to conceal the granite over an area larger than that occu- 

 pied by the sierra. In the northeastern portion of the Seriland map over 

 a dozen buttes are shown rising sharply from the subdesert plain, and 

 though these lie almost in the delta of Rio Bacuache, analogy with simi- 

 lar buttes a few miles further northeastward, which were carefully exam- 

 ined, indicates that they are not scattered island summits, as their 

 appearance suggests, but merely knobs rising from a baseleveled plain 

 of granite traversed somewhere by an ancient valley a few hundred feet 

 deep. Again, 5 miles southeast of Puerta Infierno the tide-carved coast 

 cuts a typical granitic butte a hundred feet high and as many yards 

 across, rising sharply from the inclined foot-slope, of Sierra Seri, yet the 

 rugged-faced knob is seen to surmount a granite pediment nearly half a 

 mile across in the line of section. 



It is to be remembered that the surface and structure of the district are 

 known only through two expeditions, each traversing it twice ; but so far as 

 the observations go they indicate that the vast plains diversified by scat- 

 tered sierras and buttes do not represent an alluvium-buried mountain 

 land so much as a planation level with a few monadnocks still surviving. 



Geology. — The structural geology of the Sonoran district is too little 

 known to warrant detailed description. In a general way the rocks ap- 

 pear to be (1) ancient granites, (2) partially metamorphosed limestones, 

 shales, etcetera, and (3) moderately old igneous sheets with associated 

 tuff beds. 



The granite is sometimes quite schistose, again massive and homo- 

 geneous, frequently cut by veins and dikes. Where it reveals structure 

 the trends are approximately meridional. Originally (so far as can be 

 judged) it was a floor on which the sedimentaries were laid unconform- 

 ably; now it is deformed chiefly by upbending into meridional ridges 

 with culminating nodes forming the. nuclei of sierras. There are no data 

 concerning its age except the structural relation showing it to antedate 

 the clastic strata. 



The limestones, shales, and other elastics are apparently of consider- 

 able aggregate thickness, limestones predominating in the south and 

 shales in the north. Commonly they are highly inclined ; often they are 

 folded, somewhat Appalachian-wise, into meridional ranges greatly re- 

 duced by degradation. Sometimes, as in Sierra de Tonuco, and also in 

 Sierra de Caborca (where the trend is locally latitudinal), the limestones 

 seem to be only slightly inclined. Commonly the rocks are decidedly 



