May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 220 
From this the norm calculated in the three ways described on 
page 205, is as follows: 
Composition as 

Quantitative affected by kao- Composition of 
composition. linite and limonite. unaltered rock. 
MRE EA oxo.) ae pa 28.26 24.80 21.67 
BPreRIOIAGG, «oo. fufiawia.c hs Lie 40.03 40.03 40.03 
RUBE retry fF [5 So ohoe Se oa oer ee the II.00 II.00 17.78 
BPRERRTCE Rl hl sO RU NS as. 2 Wo na EX Ayo 
rot: aes (1 se ae ei 2.96 ja), ne: 
rs) SS HAtES 7.46 3, te 
Pie persuhene<~. Gol. oe vs vam. BEN ay: 3.02 3502 
Bigetia Ch bG ss Oa sc Ava: Leland he 4.59 ey, 4.59 
ROTATE hake a on alee pale Saat 1.66 eos 
PEG aR hhh is He a = one w 0.31 ole 
Res Gis. SARS ag a 1.28 Mabe eae 
IEP PULCt tt Roe eo ware ia i en org 0.19 0.19 
Miner OXIdeS:. 5 ye ja: oe, 0.54 0.54 0.54 
99.41 99-41 99.83 
In either case the rock belongs to Class 1, Persalane, Order 4, 
Brittanare, Rang 2, Domalkalic, and Subrang 2, Dopotassic, Dellenose. 
As compared with the rock described just preceding this differs 
chiefly in being more highly feldspathic, less acidic, and in contain- 
ing more potash and ferric oxide. Its content of an appreciable 
quantity of antimony, lead, copper, zinc, and cobalt is also a remark- 
able feature. Whether the presence of these indicates an impreg- 
nation of the rock subsequent to its eruption, by waters containing 
these metals in solution, or whether they were original constituents 
of the magma, it seems impossible to say. 
The age of the rocks to which the series of the Cerro Mercado 
belongs has been stated by Ordofiez to be late Tertiary,* and this opin- 
ion is generally concurred in, since there is no doubt that the main 
elevation of the Mexican Cordilleras took place in Eocene time, and 
that the eruption of the rhyolites about Durango was subsequent 
to this. 
Being formed, therefore, in so late a period in geological history, 
the Cerro Mercado is unique as compared with other extensive iron 
deposits, such as those of Sweden and the Lake Superior region in 
North America. These, as is well known, are considered pre-Cam- 
brian in age. De Launay has called attention to this latter fact,f 
as indicating that these deposits owe their origin to conditions prevail- 
ing in the earlier stages of the earth’s history, when the crust was 
relatively thin. He concludes that the processes of their forma- 
tion must have been deep-seated in character, because the products 
*Bol. Inst. Geol. de Mexico, Num. 14, p. 65, et seq. 
+Annales des Mines, rome ser. Tome IV.  p. 50. 
