GEOLOGY OF THE PINALENO SYSTEM. 141 



Plate XI, fig. 2, exhibits an outline of one of these ranges, from north to south, and fig. 3 gives 

 a section in which the felspar dyke rises through the mass and forms the crowning pinnacles 

 and spires. 



The western or granitic portion of this elevated country unites on the left or south hank of 

 the Gila to form a lofty granite range, the Sierra Catarina, which preserves its southeast trend, 

 crossing the San Pedro about 65 miles above its mouth and blending in at that point with the 

 foot hills of the Sierra Calitro, the chain which lies on the east bank of the San Pedro. Another 

 range lies east of this latter, and is connected with it, where the Gila river breaks through the 

 range, but separates, as it passes south, to form the Piiialeno hills, which contain mountains 

 TurnbuU and Graham. This is the loftiest of the parallel chains, and is continued further south, 

 where it is met with as the Chiricahui mountains, and as far as Guadalupe. It is, however, 

 but one of three ranges — having the same strike, the same structure, and upraised by the same 

 cause, and the whole may therefore be classed together as one mountain system. 



The Pinaleno system. — The Sierra Catarina comprises the primary and volcanic rock of this 

 system, which, united into one mountain mass south of the Gila, is on the north side of the 

 river spread out into several ranges of granitic and felspar porphyry rocks, preserving an uni- 

 form northwest trend. Trachyte, red porphyry, and basalt, are the intruding rocks in the granitic 

 hills ; the trachyte in many places being spread out like a bedded rock over the sandstone, 

 which it has rendered metamorphic. 



While, as has been stated, the trend of these hills is northwest and southeast, it would appear 

 as if the force was more energetically exerted several miles north of the river, or at least exerted 

 more in parallel lines than over the whole surface ; that it diminished south and eastward, and 

 that the course of this river was near the southern termination of the wave force which ruptured 

 the crust and extended the felspar and lava rocks. The chains which come from the north have 

 their northern extremity elevated several hundred feet above the southern end ; and while the 

 mountains in their passage south drop down, the table land rises until 90 miles south, about 

 latitude 32°, the elevated mesas of Sonora and Chihuahua are reached, and the mountain 

 ranges of the north appear to exist only as isolated hills rising out of the general plain. 



The ranges to the east of the San Pedro are less contorted, and the stratification less inclined ; 

 the hills being loftier, allow also better examination of the strata underlying the basalt. Fio'ure 

 4, plate XI, shows the disposition of the strata at camp July 7. 



The sandstone which is exposed on the western faces of these hills being capped by basalt and 

 red amygdaloidal porphyry, has here about 500 feet in thickness uncovered. The total height of 

 the range on the right bank may be 2,000 feet ; dip of the sandstone 25° south, 75° east. At its 

 base is the river bed of alluvial sands and clays, and through which the Gila has cut thirty 

 feet. A low mesa, or terrace, is on each of the rivers, formed of gravel, the decay of the sand- 

 stone rock, intermingled with the wash from the primary rocks on the left bank of the stream. 

 Of these gravel and river stones, three out of four are porphyritic ; the rest granitic and feld- 

 spathic. Only two varieties found in the stream are not local : one a deep red (stilbite) porphy- 

 ritic rock, with minute crystals of hexagonal^reen mica interspersed, and the others are encri- 

 nital limestone, grayish in color, and aknost metamorphic. These were carried down by some 

 of the northern tributaries, and are, perhaps, derived from outliers of the MogoUon system. 



On the south (left) bank of the river the sandstone again appears, dipping in the same 

 direction ; but at a higher angle, 28° to 30° — it reposes on a mass of erupted lamellar feldspar. 



