127 



To tlie eye of the casual observer the foliaceous axils of the 

 leaves in particular present the appearance as if two pairs of 

 stipules existed ! (by some hocus pocus) of which upon closer examin- 

 ation one pair proves to be two conspicuously developed leaves 

 with a third rudiment, leaving attached to the stem its proper single 

 pair of broad foHaceous stipules. 



In the general appearance of the umbel of purpHsh flowers, it 

 reminds one of Astragalus hypogloUis^ but with the flowers smaller. 



August 5, 1861. 

 President in the Chair. 

 The following paper was read by Mr. R. Pumpelly : 



MlNERALOGICAL SkETCH OP THE SiLVER MlNES OF ARIZONA. 



Arizona proper, or the Gadsden Purchase, is that part of our frontier 

 which has the Rio Grande and the Colorado rivers for its eastern 

 and western, and the Gila river and Mexican boundary line for its 

 northern and southern limits. 



It thus extends over both slopes of the Sierra Madre^ which here 

 loses its continuous character, giving rise to almost unconnected 

 mountain groups. It is also traversed from N. W. to S. E. by 

 granitic sierras seldom over seventy to ninety miles in length, and 

 distant from each other from twenty to forty miles. This configura- 

 tion gives rise to a most remarkable occurrence of paralleUsm. 



The intervals between these ranges are plains, having a gradual 

 descent from the sierras on either side. In the western part of the 

 territory, where but little rain falls, water courses are very rare and 

 the surface of these tracts is almost unbroken ; but in the central 

 portion, near the larger mountains, they present the appearance of 

 extensive valleys and are cut up by river beds and frequent trib- 

 utary caTwns. 



These plains are all connected and form members of the immense 

 quaternary deposits, extending from the Gulf of California eastward. 



The quaternary formation is stratified, and composed of both 

 rounded and angular rocks with pebbles and sand — the detritus of 

 the neighboring mountains and the underlying formations. A grad- 

 ual and regular descent of the surface of the whole quaternary area 

 towards the Gulf of Cahfornia and the Colorado river is perceptible, 

 showing that there has been a gradual elevation extending over a 

 large area, and probably during a long lapse of time. That this 



