Mineralogy and Geology. 263 
erous and contain veins of silver ores. Them metamorphic slates and lime- 
stones of the Altar and Magdalena districts, which include the richest 
sibly b 
y also be noticed 
the gold which they furnish does not resemble that ohtsitied rata 
e Triassic strata. 
The Cretaceous period is also i aie at the foot of the Sierra 
Ma ides at Arivechi, in Sonora. The strata belonging to this series are 
chiefly argillaceous shales, and they ae upon porphvries and Carbonif- 
erous limestone. ey have been disturbed and elevated since their de- 
position. The fossils, which they contain in ene number and in a fine 
state of preservation, will be noticed farther o 
€ - the sii nats formations were ‘oleate in existence before 
= indications of ancient craters or vents. 
“The lithological character of the eruptive materials is extremely va- 
ried, and there seem to have been several periods of igneous action je 
s many disturbances of the strata, all of which took place 
after the close of the Cretaceous e ee Three different series of vol- 
5 
less uplifted since their deposition, - coverin 
sedimentary formations as well as older volcanic porphyries. 
attain a ~~ thickness, between San ahisks and San Ignacio, in Duran- 
go and Sin 
“ Above hens formations occur ancient alluvial od with bones of 
oo | 0 Di by Mr. 
aan 
m. Besides these, stcpe othe 7 
Shum, and Turbinolia Texana Con. ag veg p. 11. 
SS suhee. § 
