210 FIELD COLUMBIAN MusEUM—GEOLOGY, VOL. II. 
THE CERRO MERCADO. 
The Cerro Mercado, or Iron Mountain, is a hill largely made up 
of solid iron ore. It is situated three-quarters of a mile northeast of 
the City of Durango. In form it is elongated, with a roughly tri- 
angular outline, the base of the triangle lying towards the west. Its 
general trend is nearly east and west, or more exactly, as given by 
Witherbee,* N. 83° E. The hill rises abruptly above the alluvial 
plain upon which the City of Durango is situated, to an average 
height of about 300 feet, with single peaks 50 to 10o feet higher. 
The length of the hill is about one and one-third miles, and its 
average width about one-third of a mile. At about the middle of its 
extent in an east and west direction, an inset of eruptive rock cuts 
it practically in two, although the iron ore may be in contact below. 
The black color of the iron oxide, of which the hill is composed, 
is in striking contrast to. the yellow and green of the surrounding 
plain. The hill is almost bare of vegetation, except for straggling 
cacti, and its outline is bold and rugged. Steep cliffs ten to twenty 
feet high, and even higher, are not infrequent, and in places exhibit 
a distinct columnar structure like that of basalt. Such cliffs are com- 
posed wholly of ore. They bear no vegetation. From these pro- 
jecting masses of iron ore, talus slopes lead, as a rule, down to the 
level of the plain. These slopes are generally rather steep. The 
talus consists largely of fragments of the ore intermingled with soil 
and fragments of other rocks. Among the ore fragments occur 
some described by Witherbeet as being rounded and nodular, and 
imbedded in stiff, clayey matrix. Such have probably been water 
worn. In a later articlet this author states that the nodular ore 
extends to the east for over a mile, and to the south it was found 
three and one-half miles distant at a depth of 60 feet when digging 
for water. At this point it was embedded in a gravelly clay. There 
can be little doubt that these fragments have been distributed by 
the same fluviatile agencies which deposited the gravel. 
The iron oxide, which forms the great mass of the mountain, 
has often been described as consisting chiefly of magnetite. This 
is the affirmation of Weidner§ and Chrustschoff,| although the re- 
*Trans. Am. Inst. Min. Eng., Vol. XXXII., p. 156. 
Loc. ctt. 
{Mining World, March 12, 1go04, p. 21. 
§Report to the Mexican Minister of the Interior. Translated by Burkart, 
Neues Jahrbuch, 1858. 
|| Einiges uber den Cerro del Mercado, Wurzburg, 1879, p. 7. 
