VOLCANOES OP MEXICO. 23 



general desigMiation of Sierra Madre. In their central parts they consist chiefly 

 of granites and syenites, but sedimentary formations are also largely represented, 

 especially by a carboniferous limestone interspersed with thin deposits of anthracite. 



As in the Lower Californian Mountains, igneous eruptions have occurred at a 

 great many points, and vast expanses on the plains and slopes of the hills are 

 covered with molten lavas. One of the cones is not even yet quite extinct, the 

 Pinacate volcano (5,450 feet), which lies beyond the Sierra Madre proper, some 60 

 miles east of the Colorado estuary. In the middle of a vast lava field stretching 

 south of the mountain, rise a few secondary cones, one of which is pierced by a 

 cave from which escape copious sulphurous exhalations. To the genius of the place 

 the neighbouring Indians bring propitiatory offerings of shells, darts, and the like. 



The mean altitude of the Sonora Mountains scarcely exceeds 5,000 feet, but 

 some of the spurs projecting westwards rise much higher near the coast, where 

 they present an all the more imposing aspect that they are here visible from base 

 to summit, with their terminal cliffs and escarpments springing from the level of 

 the sea. Such are, near the Arizona frontier, the Sonoala highlands, one of whose 

 peaks has an elevation of 9,500 feet. Such, also, the Alamos, or " Poplar'' group 

 (5,900 feet), in the south of Sonora, followed by other coast ranges in Sinaloa. In 

 winter their lofty crests are streaked with snow, and all of them contain numerous 

 silver lodes irregularl}^ crossing each other in all directions. 



South-east of Sonora the Sierra Madre rises gradually, while still retaining the 

 same geological formation and general aspect. Here the Cumbre de Jesus Maria, 

 in the Tarahumara uplands, excteds 8,240 feet, and the Frailecitos peak, near 

 Batopilas, is said to fall little short of 9,9U0 feet. As they increase in height the 

 crests draw continually nearer to the coast, and thus present more precipitous 

 flanks towards the sea. From the coast lagoons and dunes the horizon is bounded 

 by a long line of lofty crests penetrating into the zone of clouds and vapour. 



The line of these crests and of the so-called biifas, or jagged heights, develops a 

 continuous chain at a mean distance of about 60 miles from the sea. Several of 

 its summits exceed 10,000 feet, while the Cumbre Pinial, in the Sierra del Naj^arit, 

 attains an altitude of 12,350 feet. But farther south the outer terrace of the 

 Mexican tableland, and the mountains dominating it, lose all apparent regularity 

 in their general outlines. The groups, connected togt-ther by passes at different 

 elevations, have no longer a uniform direction, and here the loftiest ridges, all 

 noted for their extremely rich argentiferous deiDOsits, lie more to the east ; south- 

 wards the whole system is interrupted b}^ the deep valle}^ of the Pio Lerma. 



Immediately opposite this breach and about 60 miles seaward rises the insular 

 chain of the Très Marias and the San Juanito, which are disposed in the direction 

 from north-west to south-east, parallel with the main continental range. In these 

 islands the highest cone, 2,430 feet, has been the scene of volcanic eruptions. 



Nor were volcanoes formerly absent in the section of the Sierra Madre which 

 lies to the north of the Pio Lerma. In several places are still seen lava fields, 

 some destitute of vegetation, others forest-clad. Here also rise mounds of scoriœ 

 and ashes, and the Breila district especially, which stretches south of Durango. is a 



