MCGEE] 



GEOLOGIC HISTORY OF SONORA 31 



ejectameuta, as at Pico Piiiacate, in northwestern Sonora. The various 

 rocks are usually bare or raeagerly mantled with talus in the niouu- 

 tains; over the greater part' of the plains they are commonly veneered 

 with sheetflood deposits, ranging from a few inches to a few yards in 

 thickness; while the central portions of the larger valleys are lined 

 with alluvial accumulations reaching many hundreds of feet in thickness. 



The clearly interpretable geologic history began with extensive 

 degradation and eventual baseleveling of a granitic terrane in Paleo- 

 zoic or early Mesozoic time; then followed the deposition of the shales 

 and associated limestones during the later Mesozoic; next came eleva- 

 tion, accompanied or followed by corrugation, chiefly in folds parallel 

 with the present coast, whereby the granite-based sierras were pro- 

 duced, and accompanied also by the earlier vulcanism to which the 

 volcanic sierras owe their existence. A vast period of degradation 

 ensued, during which the land stood so high as to induce greater precip- 

 itation than that of today and to permit the streams to carve channels 

 far below the ))reseiit level of tide, and during which the present gen- 

 eral configuration was developed; then came the southwestward tilting 

 and consequent climatal desiccation, the filling of the deeper valleys, 

 the inauguration of sheetflood erosion, some local vulcanism, and the 

 progressive shifting of the divides. 



The geologic structure affects the hydrography, especially that factor 

 determined by subterranean circulation, or ground-water; for the 

 superficial sheetflood and alluvial deposits are highly pervious and 

 many of the volcanics hardly less so, while the shales and limestones 

 are but slightly pervious and the granites nearly impervious. The 

 geologic structure also determines the character of the soil with excep- 

 tional directness, since the dryness of the air and the dearth of vegeta- 

 tion reduce rock decay to a negligible quantity. The characteristically 

 precipitous siei-ras and cerros are of naked ledges, save where locally 

 mantled with a mechanical debris of the same rocks (much finer than 

 the frost product of colder and humider regions) ; the soil of the normal 

 plains is but the little-oxidized upi)er surface of sheetflood deposits 

 made up of the mechanical debris of local rocks and varying iu coarse- 

 ness with the slope; while the soil of the valleys is detrital sand and 

 silt, derived from tributary slopes, passing into adobe where conditions 

 are fit, and essentially mechanical in texture and structure save where 

 cemented by ground-water solutions at the lower levels. 



Flora 



The flora of the Sonorau province aflords a striking example of tbe 

 adjustment of vegetal life to an unfavorable environment. The pre- 

 vailing vegetation is perennial, of slow growth and of stunted aspect; 

 and it is not distributed uniformly but arranged iu separate tufts or 

 clusters, gathering into a nearly continuous mantle iu wetter spots, 

 though commonly dotting the plains sparsely, to completely disappear 



