II. Occurrence of Oyster Shells in Volcanic 
Deposits in Sonora, Mexico. 
E. T. DUMBLE, 
Houston, Texas. 
The Mesozoic and Neozoic deposits of Sonora contain three great com- 
plexes of igneous rock. The older of these, which comes in connection 
with clastic sediments of the Triassic and underlying the Cretaceous, we 
have called the Lista Blanca. The second, which overlies the Cretaceous, 
has been named the Trincheras, while the latest has received the desig- 
nation from the town where it is so well exposed — Nogales. These com- 
plexes comprise massive igneous rocks, both andesites- and rhyolites, with 
agglomerates and tufaceous agglomerates, volcanic conglomerates, tuffs 
and scoriaceous lavas, interbedded with sedimentary deposits of sand, 
clay and lime. The details of these formations were given in “Notes on 
the Geology of Sonora,” Transactions American Institute of Mining 
Engineers, January, 1899. 
It is not always possible to determine to just which one of these series 
any given outcrop belongs, unless there is present sedimentary rock of 
either Triassic or Creatceous age. The rocks at Guaymas are of this class. 
In character they seem to correspond fairly well with the Trincheras, but 
we are inclined, on account of their position, to place them with the 
Nogales. These rocks form considerable hills around the city of Guay- 
mas, and also rise from the waters of the bay as islands, and on one of 
these there is, in a pinkish feldspathic material, a bed of oyster shells. 
The material is rhyolitic and the shells, which are entirely silicified, 
are completely imbedded in and covered by the matrix as though it had 
flowed around and over them as volcanic mud. The oysters are of the 
same general shape as those growing in the bay today, but so completely 
covered and altered as to be unidentifiable. 
At La Barranca the Coal Series of the Triassic is capped by a gray 
andesitic agglomerate belonging, we believe, to the Lista Blanca. In cut- 
ting a road through the material we uncovered the remains of a large 
oyster shell completely imbedded in this agglomerate. Such parts of the 
shell itself as remain are altered to calcite and not silicified, as the Guay- 
mas specimens were. 
Both localities were found by Mr. J. Owen. 
