166 STRATIFIED GRAVEL BEDS — ORGAN MOUNTAINS. 



they are all lone hills, lying northwest and southeast, or nearly so, and have a trachyte or 

 porphyry hase, with metamorphic quartz and conglomerate lying conformably upon them. 

 These strata dip southwest at a small angle, and in places vary from 100 to 300 feet thick. 

 The conglomerate is in places a solid, coherent rock, while in others it is almost a loose gravel. 

 This incoherent bed is usually horizontal, or nearly so, upon the igneous rock. Much of the 

 Sonora desert, in its north extremity, has its surface of this stratum, and may, in part, owe its 

 sterility to this loose and porous bed. 



On Big Horn mountains it is solid, and resembles, lithologically, the conglomerate on the 

 east slope of the Cordilleras, which is admitted to be tertiary ; if so, these are most probably 

 tertiary ; but as no opportunity presented of comparing them with any well known contiguous 

 beds, I have hesitated to assign them any other name or place than that of desert conglomerates. 

 Should these be classed as tertiaries, there would then be presented a curious fact, the absence 

 of palfeozoic strata from the Sierra Santa Catarina to the Sierra Nevada or the Cordilleras. 



This deep valley trough, where examined, shows gravels and tertiary stratified beds lying 

 unconformably to the granite rock, with which they are in contact, but possessing no formation 

 of earlier date than the Eocene period. 



This holds good along the travelled route over the Colorado desert and along the lower por- 

 tion of the Gila, where the granites are upheaved every twenty miles, intruded by trachytes and 

 lavas, without any secondary rock until the Devonian sandstones of the Calitro hills are reached. 

 In the Mojave valley, however, a mass of metamorphic limestone was met with upraised by a 

 porphyry protrusion. The calcareous rock was probably of secondary formation. 



Many of the lesser hill ranges just adverted to pass north of the Gila, and continue their 

 northwest course until they reach the Colorado or the Santa Maria, cutting up that otherwise 

 plane and desert country by a series of isolated ridges similar to those on the Sonora desert to 

 the south. 



GEOLOGY OF THE OKGAN MOUNTAINS. 



Keviewing the structure of the Organ mountain range and the valley of Mesilla, it appears 

 that the central axis is of felspathic and syenitic rock, cut through by trap and porphyritic 

 felspar, and that the earliest sedimentary rocks found are the limestone and grits of the coal 

 period. The limestone, full of encrinital casts, with a few moulds of productus and posidonomya, 

 lies in immediate contact with the syenitic rock, and conformable to it. 



On the Mesilla valley side of the Organ mountains, the order and thickness of the beds met 

 with are : 



Syenite and porphyry (leptinite.) 

 Limestone, 1,000-1,200 feet. 



owis gri , I jj^ valley — covered by cretaceous sand and alluvium. 

 Red sandstone, ) 



On the other side of the basin, at Dona Ana, there is met : 



Porphyry and leptinite. 



Limestone, 1,600 feet. 



Black shale, 50 feet. 



Yellow white grit, 130 feet. 



Reddish sandstone, 300 feet. 



The black shale found on the Dona Ana range was not observed at the Organ mountains ; as 



the limestone was the only rock elevated above the plain, it is probable the shale was covered 



