THE PLAINS ARE PLANED ROCK. 91 



of the valley-plain area is not alluvium, but planed rock similar to or 

 identical with that constituting the mountains. To the student of geo- 

 morphy this is the striking characteristic of the Sonoran region — the moun- 

 tains rise from plains, but both mountain and plain (in large part) are 

 carved out of the same rocks. The valley interiors and the lower lowlands 

 are, indeed, built of torrent-laid debris, yet most of the valley area carries 

 but a veneer of alluvium so thin that it may be shifted by a single great 

 storm. Classed by surface, one-fifth of the area of the Sonoran district, 

 outside of the Sierra and its foothills, is mountain, four-fifths plain ; but 

 of the plain something like one-half, or two-fifths of the entire area, is 

 planed rock, leaving only a like fraction of thick alluvium. This relation 

 seems hardly credible. During the first expedition of the Bureau of 

 American Ethnology (in 1894) it was noted with surprise that the horse- 

 shoes beat on planed granite or schist or other hard rocks in traversing 

 plains 3 or 5 miles from mountains rising sharply from the same plains 

 without intervening foothills ; it was only after observing this phenome- 

 non on both sides of different ranges and all around several buttes that 

 the relation was generalized, and then the generalization seemed so far 

 inconsistent with facts in other districts that it was stated only with cau- 

 tion even in conversation. During the 1895 expedition a skilled student of 

 geomorphy (Mr Willard D. Johnson) was added to the party, partly in the 

 hope that observation in this direction might be extended and verified or 

 corrected. Even then the observations and inferences seemed hardly 

 worthy of trust until the shores of the gulf of California in Seriland were 

 examined and the superb section from Punta Ygnacio to Puerta Infierno 

 was found to exhibit clearly the inferred relation. "A quarter of this 

 15-mile exposure is the current-built point, another quarter cuts butte 

 or range of igneous rock or ancient granite, while the remaining half 

 traverses typical inter montane plain in cliffs of 20 to 50 feet, and fully 5 

 out of the 7 J miles of the low cliff reveal the substratum of planed granite 

 beneath a torrential veneer, while there is more of alluvium-free granite 

 than of graniteless alluvium.' 1 * Sierra de Tonuco, lying a few miles 

 outside the northeastern corner of Seriland, f is a typical mountain mass 

 of the Sonoran district ; it is a deeply furrowed block of semi-metamor- 

 phic limestone resting on an inclined table of slightly schistoid granite ; 

 its crest is perhaps 2,500 feet above the plain, and its upper three-fifths 

 is limestone ; its base appears to be heavily burdened with taluses of 

 limestone and granitic debris, but occasional arroyas cut through these 

 aprons and show that they are not of great depth. These rubble-cum- 

 bered slopes pass within a fraction of a mile into a lowland plain so 



* National Geographic Magazine, vol. vii, 1896, pp. 127, 128. 

 t Map of Seriland, op cit., pi. xiv. 



