CONFIGURATION INDEPENDENT OF STRUCTURE. 93 



metamorphosed, the limestones into marble, the shales into phyllitic and 

 other schists, slates, etcetera ; yet sometimes, as in the sierras just noted 

 the alteration is slight. In Sierra de Caborca the limestone is highly 

 silicious (intermixed and interlaminated with silicious silt) and weathers 

 brown, highly vesicular, and ferruginous, so as to be mistaken locally for 

 lava. Collectively the clastic rocks seem to be corrugated extensively, 

 and the harder beds, together with the nucleal granite ridges and nodes, 

 form the scattered sierras characteristic of the district; but the region 

 has suffered profound degradation whereby much, probably the greater 

 part, of the clastic mass has been carried away. Locally, in certain of 

 the sierras, the topographic configuration expresses the structure, as 

 Johnson's surveys admirably show ; throughout the plains and fully 

 half of the mountain area (i. e. } in nine-tenths or more of the district) 

 there is no visible relation between structure and configuration. No 

 general faulting and monoclinal deformation such as is considered char- 

 acteristic of the Great Basin was detected. The age of the elastics has 

 not been determined, but there is some reason for correlating them with 

 the sedimentaries of eastern Mexico and certain of the metamorphics of 

 California, and regarding them as Jurassic and Cretaceous. 



In parts of the district igneous rocks and tuffs prevail, concealing the 

 ancient granites and the elastics and forming most of the buttes and lower 

 ranges as well as portions of the greater sierras ; and elsewhere similar 

 materials are found here and there, sometimes throughout considerable 

 areas. Sometimes the igneous rocks are apparently involved with the 

 elastics, and in general they seem to have suffered degradation conform- 

 able with that of limestone and schist and granite, so that the origin of 

 the greater part of them would seem to date back nearly to the close of 

 the principal deposition period. Occasionally newer looking and less 

 eroded lavas and tuffs are found, and in the western part of the district 

 there are coulees and lapilli beds, particularly about the volcanic peak 

 Pinicate ; but modern or even late Tertiary vulcanism would seem to be 

 quite local and exceptional. 



Hydrography. — Viewed as a unit, the Sonoran district is a great plain 

 sloping from the high Sierra to the sea and relieved here and there by 

 minor ranges and masses usually trending in north-south directions ; but 

 viewed in greater detail it may be considered by the student, as it is by 

 the Indian, Mexican, or American layman, as an assemblage of drainage 

 basins or u valleys." In a general way the " valleys " lie between the 

 parallel ranges, those north of the subcontinental divide (which coincides 

 approximately with the international boundary) sloping toward the Gila, 

 those beyond the divide sloping southward ; but in passing westward each 

 " valley " lies lower than its neighbor, and in the western part of the dis- 



