CRETACEOUS AND GYPSEOUS BEDS OF SAN PEDRO VALLEY. 143 



tutes their base, and is limited in surface extent by that ridge upon the east. On the west it 

 reposes unconformably upon the granitic elevation of the Santa Catarina hills, but is separated 

 from immediate contact by the bed of sandstone conglomerate, represented in figure 5, plate XI, 

 where the sandstone is marked c. 



The gypsiferous series met with in passing up the San Pedro are of great thickness ; of these, 

 the lower beds are encountered three miles above the mouth of the river, the upper beds sixty 

 miles up stream. The same series are met with by leaving the river bottom and ascending the 

 mesa towards the Arivaypa hills. They are as follows, commencing from above : 



Thickness. 



A. White marly rock, soft and friable, wearing into holes 80 feet. 



B. Greenish, aluminous rock, hard and slaty ; anhydrite 10 " 



C. Brownish gypsiferous clays 100 



D. Yellow sandstone, with seams of selenite 4 to 12 inches thick 100 



E. Conglomerate of igneous pebbles, syenitic, porphyritic, and jasper 60 



F. Greenish arenaceous gypsiferous rock 60 



Total thickness 410 feet. 



The upper beds are marly clays ; the lower, grits and conglomerates. It has already been 

 stated that these beds are unconformable to the Santa Catarina hills, whose lamellar felspar 

 dip easterly (4°) on its eastern slope. At Tucson, gypseous rock is also found dipping slightly 

 west; so that these beds appear to have been deposited on the slopes of the Santa Catarina, 

 whose elevation is thus less recent than these gypseous beds. It is remarkable that here these 

 gypseous beds repose on conglomerate, and this latter on a primary bed. This is the same 

 disposition which occurs upon the eastern slope of the sierra emerging from Vallecito. The 

 gypsum beds there present also the same physical and chemical characters with those observed 

 here. A section is given in figure 6, plate XL 



There can be no doubt that these beds correspond to the cretaceous strata of Texas and those 

 east of the Alleghany range. On the San Pedro they repose on granite, and eighty miles up 

 the river are covered up by the tertiary conglomerates and gravels which constitute the great 

 desert of Sonora and of the basin. Lower down these, having been denuded, allow the exposed 

 beds to appear. Like all other cretaceous beds, they are gypseous and marly above and 

 sandy below ; but they are singularly sparing in fossil remains, a few pieces of silicified wood, 

 washed down by a creek, being the only traces of organization observed. 



An analysis of the stratum A is subjoined in the chapter on Chemical Analysis, containing 

 one-third its weight of clay and nearly one-half carbonate of lime ; this gives to the whole mass 

 a chalky character and cohesiveness. It is a very fine marl. 



The stratum C is a mixture of clay and irregular formed crystals of gypsum, with minute 

 crystals of common salt (chloride sodium.) The saline particles prevail, and give a rough 

 and crepitating feel to the clay. 



About sixty-five miles southeast of the mouth of the San Pedro a crested hill of granitoid 

 rock crosses the river, through which the latter canons. This rock is the continuation of Santa 

 Catarina, which, in preserving its trend, crosses the river, whose tendency is to turn west- 

 ward ; from this point the gypseous beds disappear, being covered up, if they exist, by a thick 

 bed of loose conglomerate gravel, granitic sand and gravel, with quartz pebbles. 



