226 FIELD COLUMBIAN MusEUM—GEOLOGY, VOL, II. 
have been brought to light only after long ages of erosion. If the 
Cerro Mercado, however, owes its origin to similar processes, it is 
evident that these are capable of operating in the later periods of 
the earth’s history as well as the earlier. 
The association at the Cerro Mercado of a large amount of iron 
oxide with relatively acid rocks is another unusual feature, since for the 
most part other iroa deposits of probable igneous origin, such as those 
of the Urals and of Sweden, have basic rocks in association. The most 
important exception to this rule seems to be the deposit of Kirun- 
avara, Sweden, in connection with which De Launay notes acid por- 
phyries (quartz keratophyres), as occurring in considerable quan- 
tity 
The question of the origin of the Cerro Mercado is not one which 
has been heretofore extensively discussed. Weidner considered it 
to represent the product of a volcanic eruption,;, while Burkart 
regarded it as intrusive.t Weidner’s view of the volcanic origin 
was based chiefly o:. the tuffs found in the vicinity and the absence 
of evidences of co::tact metamorphism. Chrustschoff§ considered 
the process eruptive and that the stratigraphy showed that the iron 
oxide at two points broke through the porphyry and spread out over 
the latter. On account of certain peculiarities in the appearance 
of some of the ore lumps, however, he was of the opinion that the 
outpoured mass was, at the time of its eruption, in a “‘hot-fluid, 
watery’’ condition. Ordofez believed that the iron oxide was of a 
later origin than the surrounding rocks and filled a fissure formed 
in them.|| The supposed steps of the process are not indicated in 
any further detail by this author, however. With the exception of 
Ordofiez it will be seen that the general trend of opinion has been in 
favor of an eruptive origin for-the mass. Among earlier writers, 
however, the possibilities of sedimentary origin of iron deposits were 
less generally accepted than now. 
In favor of an igneous origin for the iron oxide of the Cerro 
Mercado, the wholly igneous character of the accompanying rocks, 
the integrity, continuity, and extent of the iron oxide, and its un- 
stratified, columnar structure, are the factors which in the view of the 
present writer weigh most strongly. 
If it be of igneous origin, however, it is difficult: to conceive 
of the steps of the process by which it was formed. It seems 
necessary to suppose that either, on the one hand, it welled up 

MOD: Ot be acre 3 TOp. cit. 
tOp. crt. SO p. cit. 
|| Bol. Inst. Geol. de Mexico, Num. 16, p. 11. 
