CRETACEOUS AND GYPSEOUS BEDS OF SAN PEDRO VALLEY. 
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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 u 
E. Conglomerate of igneous pebbles, syenitic, porphyritic, and jasper..................... 60 u 
F. Greenish arenaceous gypsiferous rock................................ 60 11 
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 Yallecito. The 
gypsum beds there present also the same physical and chemical characters with those observed 
here. A section is given in figure 6, plate XI. 
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 0 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. 
