292 EXCURSIONS TO THE PROVINCES. 



making astronomical observations, in studying the 

 natural productions of the surrounding country, and 

 in enjoying the society of enlightened individuals. 

 The longitude of Mexico, which had been misplaced 

 two degrees on the latest maps, was accurately 

 determined by a long series of observations. 



Our travellers next visited the celebrated mines 

 of Moran and Real del Monte, and examined the 

 obsidians of Oyamel, which form layers in pearlstone 

 and porphyry, and were employed by the ancient 

 Mexicans for the manufacture of knives. The cas- 

 cade of Regla, a representation of which forms the 

 vignette to the present volume, is situated in the 

 neighbourhood. The regularity of the basaltic col- 

 umns is as remarkable as that of the deposites of 

 Staffa. Most of them are perpendicular; though 

 some are horizontal, and others have various degrees 

 of inclination. They rest upon a bed of clay, be- 

 neath which basalt again occurs. Returning from 

 this excursion in July, 1803, they made another to 

 the northern part of the kingdom, in the course of 

 which they inspected the aperture made in the 

 mountain of Suicog for the purpose of draining the 

 valley of Mexico. They next passed by Queretaro, 

 Salamanca, and the fertile plains of Yrapuato, on the 

 way to Guanaxuato, a large city placed in a narrow 

 defile, and celebrated for its mines. 



There they remained two months, making re- 

 searches into the geology and botany of the neigh- 

 bouring country. From thence they proceeded by 

 the valley of San Jago to Valladolid, the capital of 

 the ancient kingdom of Mechoacan ; and, notwith- 

 standing a continuance of heavy autumnal rains, 

 descended by Patzquaro, which is situated on the 

 edge of an extensive lake towards the shores of the 

 Pacific Ocean, to the plains of Jorullo. Here they 

 entered the great crater, making their way over 

 crevices exhaling ignited sulphuretted hydrogen, and 



