CURRENT-DIVERGENCE IN PIEDMONT RIVERS. Ill 



whose climate and configuration approach those of Papagueria. It may 

 be noted as probable, also, that even in the more humid, provinces a 

 process analogous to that of sheetflooding may exist; for wheresoever 

 rain falls the waters gather into a moving film before rivulets and streams 

 are formed, and this film must be a more or less active geologic agent. 

 Finally, it may be noted that certain obscure phenomena of various 

 waterways which, like the Susquehanna and other Piedmont rivers in 

 parts of their course, tend to corrade their channels laterally rather than 

 vertically, appear to be akin to those of sheetflooding. 



Explanation of Plates. 



Plate 10. — Baboquivari Peak, looking eastward from Fresnal. 



The summit of the peak rises about 4,500 feet above the plain in the foreground. 

 So far as seen, the rocks of the range are ancient granites, passing into dark-colored 

 schists near the western base. The conspicuous features of the sierra are steepness, 

 ruggedness, and the dearth of taluses and foothills ; the peak has never been 

 climbed. The baselevel plain forming the foreground and middleground is excep- 

 tionally irregular, rising toward the right into a smaller counterpart of Fresnal hill 

 (which is located directly in the rear of the point from which the photograph was 

 taken). The surface deposit of the plain, as shown in the immediate foreground, 

 consists of granitic debris in the form of angular boulders and pebbles, sharp sand, 

 and silt; it usually ranges from two to five feet in thickness. The under-rocks of 

 the plain, as shown in a deep arroya and its tributary gulches just beyond the field 

 on the left, are dark schists and granitoid materials, similar to those forming the 

 adjacent base of the sierra. The view illustrates the abrupt transition from moun- 

 tain side to plain. The air-line distance to the nearest salient is about three miles 

 and to the crest of the peak about five miles. 



Plate 11. — Poso Verde Plain, looking southeastward. 



The middleground represents part of the Papago Indian village of Poso Verde 

 and, immediately beyond, the arroya forming the extension of a barranca heading 

 in the mountains on the left. The central part of the picture shows the great tor- 

 rential plain sloping southward to Rio Seco, on which sheetnood traces were found 

 in 1891. The immediate foreground illustrates the usual relation between moun- 

 tain-side and plain, the mountain rocks consisting of granite and the plain deposits 

 of granitic debris. At this point the debris mantle is exceptionally thick (15 to 18 

 feet), as shown by the excavation about the spring supplying the village, but a few 

 hundred yards down the slope toward the left granites are exposed within five feet 

 of the surface of the plain, and the great arroya in the middleground bottoms on 

 granite often rising nearly to the surface. The mountain spur forming the horizon 

 left of the center is less rugged than usual, and its curved profile blending with that 

 of the plain is exceptional — it is a nearly isolated mass affording little space for the 

 gathering of waters, which are free to spread widely on approaching the plain. 

 This salient is 7 or 8 miles, and the range faintly shown beyond 30 or 40 miles 

 distant. 



XVI— Bull. Geol. Soc. Am.. Vol. 8, 1896 



