MINING GEOLOGY 
91 
tral railroad, the country is deeply disected. Recent erosion often 
attains depths of 800 feet, cutting entirely through the Mid Cre- 
tacic sandstones and shales, and revealing an old denudation sur¬ 
face having for its foundation the Mid Carbonic Maderan lime¬ 
stones, which are folded, faulted, and intruded by dikes. Wher¬ 
ever the dikes reach canyon sides, or walls of the Mesa, they are 
plainly seen not to penetrate the Cretacic formations (figure 9). 
Thus an early Mesozoic date is fixed, within very narrow limits, 
for the volcanic activities expressed by the dikes. Formation of 
the iron ores appears to have occurred long afterwards. 
The time-span covered by the stratal hiatus represented by the 
unconformity plane at the base of the local Cretacic sections is one 
of very large proportions. Several thick rock successions are 
entirely missing. The entire Bernalillan series of red-beds, 1,500 
feet in thickness, part of the Guadalupan series of sandstones and 
limestones, perhaps 2,500 feet, the Cimarronian series of red- 
beds, 1,000 feet in vertical measurement, all of the Triassic se¬ 
quence, possibly another 1,000 feet, and all of the Jurassic sedi¬ 
ments, 500 feet or more, were swept away during the following 
Comanchan erosion interval. Altogether, about 7,000 feet of 
strata are destroyed during the Early Cretacic Period of exten¬ 
sive regional denudation. In places, as on the Chupadera Mesa, 
the Cretacic peneplain is now laid bare, and it is only because of 
this fact that the ancient Cretacic iron deposits are now again 
brought to sky. 
Notwithstanding the circumstance that the Chupadera iron de¬ 
posits are so venerable, and the vicissitudes through which they 
have passed are so many and varied, their geological story is 
quickly told. As shown at fresh contacts the ores are manifestly 
not mineralizations of the intrusive phenomena. The geneses of 
dike and ore are widely separated in point of time. As indicated 
by the continuity of the Cretacic strata immediately over the dikes 
the intrusions are without question pre-Cretacic features. The 
monzonite intrusions are possible at any date between Mid Car¬ 
bonic times and Early Cretacic times. It is probable that they 
really took place just previous to Comanchan erosion, while the 
mile and a half pile of Late Carbonic, Triassic and Jurassic sedi¬ 
ments were still untouched by the elements. It is may be that the 
dikes were formed while the Mid Carbonic limestones were yet 
