BASALT MESA JORNADA 135 



on Big Horn mountains and further west, which are themselves most probably tertiary. On 

 the north side of the Gila the basalt flow does not approach the river, but in many places is 

 as far removed as seven or eight miles, the intervening space being occupied by a low, sandy 

 mesa, not more than twenty feet higher than the river bottom. On the lowest portions of this 

 mesa mesquite grows well, and in the river bottom a luxuriant growth of grass and willow ; on 

 account of the width of the river valley at this point, and its being well supplied with water, 

 there is no doubt that, by irrigation, cotton, maize, yams, and all the growth of the southern 

 States, (rice excepted,) might be raised here. 



Hills of quartz conglomerate, capped with basalt and trachyte rocks, form the horizon on the 

 north or right bank of the river, having a trend north toward the Colorado ; these are also 

 repeated south toward the eastern limit of the basaltic mesa, forming hills 700 to 900 feet 

 high ; near the east limit of this overflow is a small hill 700 feet high through a caiaon in which 

 the trail runs^ and to avoid which the river makes a bend to the north. 



Trachyte and porphyritic (red felspar paste) rock formed the axis of the upheave, and on the 

 eastern side an outflow of basalt forming an elevated table or mesa land, extending from five to 

 seven miles in an easterly direction ; but as the river lay to the north, the trail left the mesa, 

 and descended to the river bottom. In ascending this hill on the west side, the slope was easy 

 and lay over thick beds of drift gravel and felspathic sand lying unconformable ; the axes of 

 the hills were composed of reddish felspar dykes and intrusion of white porphyritic trachyte 

 rock ; these are flanked by quartz conglomerate rock, stratified and metamorphic, the former 

 dipping 20° southwest. 



On passing through the caiion and reaching the east slope, the drift gravel no longer appears, 

 the trail passing over the basaltic floor again, which constitutes the mesa ; if the basalt covering 

 existed on the west side of the range, it must have been covered up by the deep layer of loose 

 conglomerate gravel. Underneath the lava is a cream and salmon colored sandstone grit. 



In the river bottoms, angular fragments of lava, (basalt,) with felspar porphyry veins, chal- 

 cedony, amygdaloidal lava, and reddish amygdaloidal felspar, semi-transparent quartz and 

 epidote. On the 23d of June the basalt mesa was ascended, and on the afternoon of the 25th 

 had cleared its eastern limit ; during this time the trail made many descents to the river bed 

 for water and to avoid the detour which the deep gashes in the flow would have rendered 

 necessary. It is upon this mesa that the hieroglyphical inscriptions occur, delineated by Major 

 Emory, (notes of Mil. Recon'ce, p. 89 ;) they also were found on the edges of the basalt plain, 

 where the trail descends to the river bed. The total width of the basalt overflow is above 

 thirty-six miles from west to east, and its extension north and south is much greater. Since 

 its outpouring it apjjears to have undergone but little alteration of position ; and the only chemi- 

 cal change is that of the weather loosening the amorphous zeolites from their small cavities 

 and washing them away, leaving the surface of the rock cellular, giving it the appearance of a 

 vesicular rock ; but it is distinctly amygdaloid and contains no air cavities. 



At the base of the trachyte hill, forming the east edge of the basalt mesa, the river again 

 spreads out into a wide bottom with lagoons, and covered with willow and mesquite, and supply- 

 ing tolerable grass ; on the upland, on each side, there is no vegetation, save that of desert 

 character, larrea and sage ; the i^italiaya (cereus giganteus) is abundant and increases in size and 

 beauty the further west the road is travelled. East of these lagoons the trail leaves the river 

 which passes north to turn a large mass of mountain, the road running east, a little south over 

 the Jornada to the Pimas plains ; this distance, forty miles, without water, is across a range 



