Doyie—Rio del Fuerte of W. Mewico, and its Tributaries. 65 
At Guaza the Chinipas and Septentrion rivers join, and there 
is a little level ground laid down by the rivers during a temporary 
obstruction of the lower gorge, caused by recent volcanic eruptions, 
to be further noticed. Between Guaza and La Junta, where the 
main river forms, the valley has been eroded to the base of the 
volcanic series, exposing syenite; a calcareous fossil was also ob- 
served in the shingle, though time did not allow a search for its 
parent stratum. 
Turning now east, up the main branch of the Fuerte river, 
we have before us a portion of country about 40 miles long and 
20 miles wide occupied by the wasted voleanoes of a secondary 
eruption ; all the rocks here are dark basic lavas, and some of the 
flows can be seen little altered, still forming the surface of the 
ground, or filling former river valleys. Mineral veins are abun- 
dant in a belt reaching from San José de Gracia near the boundary 
of Durango, to Rosario mountain, near that of Sonora, especially 
at the junction of the dyke-intersected syenite, with the volcanic 
rocks. Gold occurs frequently, associated with silver and copper 
as sulphides, and sometimes in veins of iron ore. 
As the river runs in a chasm, the practicable track crosses the 
shoulder of the Cerro de Volean—a prominent peak among the 
foothills; its top is evidently the hard core of a crater, and its sides 
the eroded materials of the cone; denudation is proceeding apace 
among these small mountains from the action of the heavy autumn 
rainfall on their loose layers; nearly all the ground stands at the 
angle of friction, everything is just ready to roll; and the tops of 
the ridges and bottoms of ravines are quite narrow. Near Volcan 
there are four or five other extinct cones in the same stage of decay. 
Descending to the bed of the river at San Francisco, one finds it 
rocky, and with a rapid fall ; numerous waterworn boulders as large 
as 10 feet in diameter are piled along the margin, attesting the 
force of the stream when swollen 50 or 60 feet above its dry season 
level. Above Realito it has cut through a thick homogeneous 
layer of reddish rock, very free from fissures, and has left perpen- 
dicular cliffs nearly a thousand feet high; on emerging from this 
pass, one stands ona lake terrace over the river ; observation of the 
surrounding hills discloses other parallel terraces at different higher 
levels. The wide valley is covered by regularly bedded lake 
deposits through which the river winds in sinuous curves, cutting 
SCIEN. PROC. R.D.S8., VOL. IX., PART I. F 
