I. Cretaceous of Obispo Canyon, Sonora, 
Mexico. 
E. T. DUMBLE, 
Houston, Texas. 
The trail between La Barranca and La Dura, Sonora, after crossing 
Obispo Pass, follows the Obispo canyon southward for three or four 
miles. In this canyon, about ten miles from La Dura, there is an expos- 
ure of thin to medium bedded limestones of light color, streaked with 
gray or black, which rest upon a purple volcanic agglomerate and are, in 
( turn, overlain by the "greenstone” of the region. 
An examination of the beds show that they form the sides of a range 
of rounded hills which extend along the west bank of the canyon for 
nearly a mile, and that they carry fossils of Cretaceous age. The fossils 
are all, or nearly all, siliceous pseudomorphs and badly preserved, and it 
required considerable search to find forms that could be positively identi- 
fied. Many oysters were found, a few gasteropods, a fair specimen of 
the form formerly known as Cypremaria, part of the plates of a large 
echinoid of the Cidaridse class and small Gryphsea — enough to prove its 
Cretaceous age. The thickness of the beds is not more than seventy-five 
feet. They rest, as has been stated, on a purple agglomerate. I was not 
able to ascertain positively whether the contact showed an unconformity 
or not, but I think it did. Just below the first limestone there is in the 
thin bedded agglomerate a bed of Hint, then agglomerate and then lime, 
the base of which was mixed with the agglomerate. At other places it 
seemed to me that the limestone rested directly on the bed of flint. 
This agglomerate is here the uppermost bed of the Triassic volcanic 
complex — the Lista Blanca, and the andesitic greenstone which immedi- 
ately succeeds the Cretaceous limestone is here the base of the Trincheras 
complex and of supposedly Tertiary age. 
The nearest Cretaceous known is that at Arivechi, sixty miles or more 
to the east, and that near Casita, more than that distance west of this 
locality. 
Besides being of importance as a new and hitherto unnoticed occur- 
rence of the Cretaceous, this place is still more important as showing 
most clearly the actual demarkation of these two systems of igneous 
rocks, and as the only place so far found where both occur in such a posi- 
tion and connection as to be definitely and accurately limited and deter- 
mined. 
