0eo0rapbical Hotes on 



GEOLOGY. 



The geology of Mexico has been but imperfectly studied. In the 

 higher ranges the prevailing formations are granite, which seem also 

 to form the foundations of the plateaus, above which rise the traps, 

 basalts, mineral-bearing porphyries, and more recent lavas. Hence, 

 Lyell's theory that Mexico consisted originally of granite ranges with 

 intervening valleys subsequently filled up to the level of the plateaus 

 by subterranean eruptions. Igneous rocks of every geologic epoch cer- 

 tainly form to a large extent the superstructure of the central plateau. 

 But the Mexican table-land seems to consist mainly of metamorphic 

 formations which have been partly upheaved, partly interpenetrated, 

 and overlaid by igneous masses of all epochs, and which are chiefly 

 represented by shales, greywacke, greenstones, silicious schists, and 

 especially unfossiliferous limestones. All these formations are alike 

 remarkable for the abundance and variety of their metalliferous ores, 

 such as silver, silver glance, copper, and gold. Gneiss and micaceous 

 schists prevail in Oaxaca and on all the southern slopes facing both 

 oceans. But the highest ranges are formed mainly of plutonic and 

 volcanic rocks, such as granites, syenites, diorites, mineral-bearing 

 trachytes, basalts, porphyries, obsidian, pearlstone, sulphur, pumice, 

 lavas, tufa, and other recent volcanic discharges. Obsidian (itzli) was 

 the chief material formerly used by the natives in the manufacture of 

 their cutting implements, as shown by the quarries of the Cerro de las 

 Navajas (Knife Cliff), near Real del Monte and Pachuca in the State 

 of Hidalgo. Vast deposits of pumice and the purest sulphur are found 

 at Huichapam and in many of the craters. But immeasurably the 

 most valuable rocks are the argentiferous porphyries and schists of 

 the central plateau and of Sinaloa, unless they are destined to be 

 rivalled by the auriferous deposits of Sonora. Horizontal and strati- 

 fied rocks, of extremely limited extent in the south, are largely devel- 

 oped in the northern states, and chalk becomes very prevalent towards 

 the Rio Grande and Rio Gila valleys. To this chalk and to the sand- 

 stones are probably due the sandy plains which cover vast tracts in 

 North Mexico, stretching thence far into New Mexico and Texas. 

 Here the Bolson de Mapimi, a vast rocky wilderness inhabited until 

 recently by wild tribes, occupies a space of perhaps 50,000 square 

 miles in Coahuila and parts of the surrounding States. 



None of the horizontal layers seem to be very rich in ores, which 

 are mainly found in the metamorphic, palaeozoic, and hypogene rocks 

 of Durango, Chihuahua, and the south. Apart from Sinaloa and 

 Sonora, which are now known to contain vast stores of the precious 

 metals, nearly all the historical mines lie on the south central plateau at 

 elevations of from 5500 to 9500 feet. A line drawn from the capital to 

 Guanajuato, and thence northwards to the mining town of Guadalupe 



