338 EXCURSIONS TO THE PROVINCES. 
misplaced two degrees on the latest maps, was accu- 
rately determined by a long series of observations. 
Our travellers next visited the celebrated mines of 
Moran and Real del Monte, and examined the obsi- 
dians of Oyamel, which form layers in pearlstone and 
porphyry, and were employed by the ancient Mexi- 
cans for the manufacture of knives. The cascade of 
Regla, a representation of which forms the vignette 
to the present volume, is situated in the neighbour- 
hood. The regularity of the basaltic columns is as re- 
markable 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, beneath which basalt again 
occurs. Returning from this excursion in July 1803, 
they made another to the northern part of the king- 
dom, in the course of which they inspected the aper- 
ture 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, notwithstand- 
ing 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 ex- 
haling ignited sulphuretted- hydrogen, and experi- 
encing much danger from the brittleness of the lava. 
The formation of this volcano is one of the most 
