107 



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 oc- 

 cupied by the wasted volcanoes of a secondary eruption; all the rocks he- 

 re are dark basic lavas, and some of the flows can be seen little altered, 

 still f ormiug the surface of the ground, or filling f ormer river valleys. Mi- 

 neral veins are abundant iu a belt reaching from San José de Gracia near 

 the boundary of Durango, to Rosario mountain, near that of Sonora, es- 

 pecially at the junction of the dyke-intersected syenite, with the volca- 

 nic 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 hest houl- 

 der of the Cerro de Volcan apromiueut peakamong the foothills; its top 

 Í8 evidently the hard core of a cráter, and its sides the eroded materials of 

 the cone; denudation is proceeding apace among these small mountains 

 from the action of the heavy autumn rainf all on their loóse layers; nearly 

 all the ground stands at the angle of friction, everything is just ready to 

 roll; and the tops of the rigdes and bottoms of ravines are quite narrow. 

 Near Volcan there are four or five other extinct coues in the same stage 

 of decay. Descending to the bed of the river at San Francisco, one finds 

 it rocky, and with a rapid faU; numerous waterworn boulders as large as 

 10 feet in diameter are piled along tho margin, attesting the forcé of the 

 stream when swoUen 50 or 60 feet above its dry season level. Above Rea- 

 lito it has cut through a thick homogeneous laj'er of reddish rock, very 

 free from fissures, and has left perpendicular cliffs nearly a thousand feet 

 high; on emerging from this pass, one stands on a lake terrace over the 

 river; observation of the surrounding hills discloses other parallel terra- 

 ces at different higher levéis. The wide valleyis covered by regularly bed- 

 ded lake deposits through which the river winds iu sinuous curves, cut- 

 ting cliffs from 40 to 70 feet high. Against the sky is the vast wall of the 

 distant mesa, indented with square notches, like themachicolatedparapet 

 of a castle, by the straight-sided ravines. 



The bed of the ancient lake is 1000 feet above sea-level at is lowest 

 point, and consists of white and bnght green sandstones and conglomera- 

 tes in thin but continuous layers. Duñng the dry season the river bed is 

 a convenient road, though in necessitates frecuent fording, as the stream 

 meanders from side to side. The sandstones were examined for fossils, but 

 none were found. After nine miles of gently-sloping, walled-in course, a 

 place Í8 reached where the lake beds hrve been cleared away nearly down 

 to the present river level: probably by wanderíngs in the course of the cu- 

 rrent: and on this plain is the small towu of Tubares. 



The ancient alke and its fluctuations of level must habe been cau* 



