32 



University of California Publications in Geology t VoL n 



Eansome 51 has described sands, with fresh feldspar grains, formed 

 by the granular disintegration of rocks under arid conditions in 

 Arizona : 



With the exception of the timbered slopes of the Pinal Mountains, and of a few 

 alluvial areas along the main arroyos, the surface of the region is almost destitute 

 of soil. The scanty shrubbery, and the sparse grass and herbage which spring up 

 with wonderful rapidity after the rains, are insufficient to prevent such soil as 

 may form from being quickly washed away. The humus acids, which in moister 

 climates and beneath a covering of soil aid in rock decay, have in this region 

 little opportunity to form or to attack the rocks. The latter crumble or flake under 

 the influence of sharp atmospheric changes, and these fragments are rapidly car- 

 ried into the valleys. The granitic masses crumble into particles of quartz, flakes 

 of mica, and angular fragments or crystals of comparatively fresh feldspar. The 

 rains acting on this disintegrated material soon wash it down to the larger streams, 

 which carry off the quartz and mica. The larger fragments of feldspar often build 

 up alluvial fans at the mouths of the small ravines heading in a granitic area, and 

 such fans are remarkable for the purity and freshness of the feldspathic material 

 which composes them, the numerous cleavage faces flashing brightly in the sun. 



Similar conditions have been described by other writers. 



The possibility that the deposition of the Franciscan sandstone 

 may have occurred in an ordinary humid climate would appear to be 

 barred by the mineralogic nature of the rock. While granular disin- 

 tegration may be produced in such climates, still the amount of chem- 

 ical decay is considerable. This question has been considered by 

 Barton, who finds that under such conditions the chemical decay of 

 feldspar is important, and the resulting deposits of arkose show cer- 

 tain characteristics : 



1. They are of small size. 



2. They grade into quartzites. 



3. The matrix will contain much argillaceous material from the 

 decomposition of the feldspars. 



4. The arkosic material will be associated with beds of argillaceous 

 material. 



5. The feldspar grains will show decomposition. 



Only the fourth criterion could be applied to the Franciscan sand- 

 stone. All its other characteristics are opposed to the idea of such an 

 origin. The presence of fresh and angular feldspars, together with 

 fresh biotite and other ferromagnesian minerals, all combine to show 

 that there was little opportunity for the decomposition which would 

 have occurred if the climate had been of the temperate humid type. 

 The wide extent of the formation is also opposed to the idea of 

 humidity. 



si Eansome, F. L., Geology of the Globe Copper District, Arizona, U. S. 

 Geol. Surv. Professional Paper no. 12, p. 21, 1903. 



