proceedings: geological society 53 



east flanks upland valleys separated from the lower canyons by steep 

 grades and falls. Such valleys have been considered by Davis ade- 

 quate evidence of renewed uplift of a fault-block mountain. Other 

 ranges may fall in this class solely because they have been inadequately 

 explored. 



The remaining mountains and groups of hills, fourteen in number, 

 are so little known that they can not be classified. Of the sixty-eight 

 ranges, forty-two are young, mature, rejuvenated or resurrected moun- 

 tains. 



The intermontane valleys are in part formed of plains cut in rock, 

 but McGee's estimate of 50 per cent of rock surface is an over-state- 

 ment. The Gila, Santa Cruz, and Altar valleys are dissected not 

 only by ephemeral streams which cut the alluvial slopes leading from 

 the mountains, but by the axial streams. In none of the sections of 

 the alluvium exposed by this dissection have lake beds been found. 

 The axial trenches of Santa Cruz and Altar valleys fade out before 

 reaching Gila River, and the terraces of the upper parts of these val- 

 leys can not be directly correlated with the terraces of the Gila. The 

 axial streams of the smaller desert valleys are not entrenched except 

 near Gila River, and the centers of these valleys are broad plains of 

 alluviation with, however, no evidence of extinct lakes. The upper 

 parts of the alluvial slopes are, however, nearly always dissected. 

 This peculiarly arranged terracing and dissection presents one of the 

 knottiest problems in the physiography of southern Arizona. On the 

 successful correlation of the several stages of dissection and alluviation 

 of these valleys will depend the interpretation of Pleistocene history 

 in the region. Solution of this problem w411 also throw light on the 

 validity of climatic terraces. 



34 1 ST MEETING 



The 341st meeting of the Society was held in the Auditorium of the 

 Cosmos Club on Wednesday evening, December 10, 1919. Mr. M. R. 

 Campbell presided. 



President E. O. Ulrich delivered the presidential address for 191 9, 

 entitled : Major causes of land and sea oscillations. 



This paper will be published later in the T^'^^pn.^L of the Academy. 



After a recess, the twenty-seventh Annual Meeting was called to 

 order by President Ulrich. The reports of the secretaries, treasurer, 

 and auditing committee were read and accepted. The balloting on 

 nominees for oflficers for 1920, proposed by the nominating committee, 

 resulted as follows : 



President, David White; First Vice-President, George W. Stose; 

 Second Vice-President, W. C. Alden; Secretaries, R. S. BasslER, 

 Laurence La Forge; Treasurer, George R. Mansfield; Menibers- 

 at-Large of the Council, C. Wythe Cooke, J. M. Hill, H. D. Miser, 

 Eugene Stebinger, R. C. Wells. 



R. W. Stone, Secretary. 



