52 proceedings: geol,ogicai, society 



similar rocks, with some basalts. The sedimentary rocks are, in part, 

 of terrigenous origin and similar to those being deposited in the present 

 valleys. There are also limestones containing indistinct fossils which 

 were deposited in large bodies of brackish water which may have had 

 connection with the sea through the Gulf of California. In the Quater- 

 nary, alluviation exceeded volcanism in importance. The sediments 

 belong to three groups: (i) an older, disturbed and partly consolidated 

 group; (2) a younger and only locally consolidated group; (3) the de- 

 posits of the present streams. 



Kirk Bryan: Geology and physiography of the Papago country, Ari- 

 zona. 



The Papago country is a large area in southwestern Arizona, lying 

 south of Gila River and west of Santa Cruz River. Part of the results 

 of a broad reconnaissance, covering nearly four square degrees, and 

 made for the purpose of preparing a guide to routes of travel and 

 watering-places in this desert region, was presented. The work was 

 done in an automobile between September 4 and December 23, 191 7. 

 The speedometer mileage was 4,250 miles; logs for 1,920 miles of road 

 were obtained; 240 photographs taken; and a topographic map con- 

 taining much new geographic information, as well as a geologic map, 

 were prepared. 



The following broad generalizations can be made: (i) The moun- 

 tains are not residual mountains in the old age stage of erosion, but the 

 majority are fault block mountains in youth and maturity, or rejuven- 

 ated and resurrected mountains; (2) the valleys are with one exception 

 drained and probably have never contained lakes. 



Fifty-four of the sixty-eight mountain ranges and groups of hills in 

 the area can be divided into three classes: 



Class I consists of 21 mountain ranges composed in large part of 

 alternating beds of lava, tuflf, volcanic conglomerate and agglomerates, 

 and stream-laid conglomerates, probably of Tertiary age. Two of 

 these mountains are old volcanoes, 10 are rather simple fault-block and 

 horst mountains, 11 are complexly faulted mountains and dissected 

 plateaus with large or small masses of the underlying basal complex. 



Class II consists of 17 mountain ranges composed largely of rocks 

 of the Basal Series: granites, gneisses, quartzites, schists, and phyllites 

 of pre-Cambrian age; felsites, both intrusive and extrusive, and gran- 

 ites of probable Mesozoic age; Paleozoic limestones; and Cretaceous 

 sandstone and shales. While largely composed of rocks of the Basal 

 Series, these mountains contain patches of lavas of Tertiary age tilted 

 and dislocated and so disposed as to indicate that uplift followed the 

 extrusion of the lava, and that the present mountains have been resur- 

 rected by the removal of a more or less continuous lava capping. 



Class III consists of 14 mountain ranges composed wholly of rocks 

 of the Basal Series, with no known association of Tertiary lavas. The 

 mountains of this class approach most closely the prevailing concep- 

 tion of the mountains of the region as old age types. Two of thetn, 

 however, the Estrella and Tinajas Altas mountains, have on their 



