ACADEMY OF NATURAL SCIENCES. 253 



character of the fossils indicates a closer relationship of the formation to the 

 eastern Cretaceous than to that of California. — j. d. w.] 



Volcanic Rocks. 



Stratified volcanic deposits cover a broad area of the surface between San 

 Ignacio and San Dimas (Sinaloa and Durango) ; they dip to the west. On the 

 other side of Durango they dip in the opposite direction. The serrated edges 

 of the strata may be seen from a great distance, and are extremely picturesque, 

 towering up like old ruins, their peculiar forms being due to erosion. They are 

 well seen in the Cerro de los Frayles, near Guarisamey,in Durango-, visible from 

 Mazatlan. These belong to the second series of volcanic rocks. In Sonora 

 there are three different series of volcanic deposits which form serrated, pictur- 

 esque, parallel ridges, running north from Guaymas as far as La Magdalena, or 

 over eighty leagues in a straight line. The three main ridges, enumerated from 

 west to east, are, 1, Las Tetas de Cabra ; 2, Guaymas de Zaragosa ; 3, Range 

 north of the mouth of the Taqui River. The description of the various vol- 

 canic deposits of Northern Mexico may be reserved for a separate memoir, as 

 the number of them is very great. 



Mines. 



The richest and widest veins are those northeast of Mazatlan, near San Dimas, 

 Guarisamey, etc., in Durango. These veins cut all the rocks older than the Cre- 

 taceous, whether igneous or sedimentary. The mines of Sinaloa are richer than 

 those of Sonora. In the former State the ore-bearing portion of the veins is 

 from a few feet to several yards in width : in the latter, generally from one to 

 two feet. In Durango and Sinaloa gold, native silver, and sulphuret of silver 

 occur associated with galena, yellow blende, and iron pyrites. In Sonora the 

 principal ores are argentiferous gray copper, with galena, black blende, copper 

 pyrites, arsenical pyrites, carbonate of lead, ruby silver, arsenical silver, and 

 gold. Each mining district is characterized by a peculiar system of veins ; in 

 all as many as twenty different systems have been observed. The most 

 abundant vein stones are quartz, either chalcedonic, crystalline, or massive ; 

 brown spar ; heavy spar ; oxide of iron. The veins occurring in the meta- 

 morphic Triassic rocks, are usually parallel with the stratification, so that they 

 lie nearly horizontal where the formation has been but little disturbed. As to 

 the yield of the silver ores, it varies extremely, and it would be necessary to 

 enter into a full description of all the different districts to give an idea of it. 

 It may be noticed, however, that the arsenical pyrites, which is auriferous in 

 the Sierra Nevada, becomes argentiferous in the Sierra Madre. The veins 

 vary in their direction from a little east to a little west of north ; the richest 

 ores near San Dimas run northeast and southwest. There are but few rich 

 mines in Sonora, a state of which the mineral wealth has been much exag- 

 gerated. There are, however, some deposits of variegated copper, and veins 

 of magnetic and specular iron. 



The annexed tabular statement will give the principal facts obtained with 

 regard to the mines examined in Northern Mexico. 



