92 
MINING GEOLOGY 
depressed two miles beneath sea-level, as the pre-Cretacic sedi¬ 
ments were being piled high upon them. 
At any rate, when this great thickness of sediments swung back 
to land, and were eroded and entirely removed during the long 
Comanchan Period, down to the Mid Carbonic limestone level, iron 
set free mainly from the decomposition of the dikes and the ad¬ 
joining rocks, segregated as limonite in the cavernous belts along 
side of the dikes. As surface limonite deposit, after several thous¬ 
ands of feet of it and the dike had been removed through erosion, 
it underwent the same vicissitudes as the old ground surface in its 
depression a mile or two below the sea, while the Cretacic and 
Early Tertic beds were being laid down upon it. At this time the 
limonite doubtless underwent complete dehydration and assumed 
the form of the oxide, magnetite chiefly. In this state it rose to 
the surface again as its cover was removed. At our day it is 
caught by us just as it was left in Comanchan times, except with 
the dehydrating impress of its long and deep burial still upon it. 
Today limonite is accumulating along side of the dikes in the 
shatter belts of the limestone just as it probably originally did in 
old Comanchan times, and just as it is in other places in the 
region. The singular admission in the Lindgren report that not¬ 
withstanding the fact that no evidences were obtainable because 
of the decomposition of the walls of iron body yet the ore is a 
contact deposit, has another angle. The real reason, perhaps, why 
the mineralized contact belt of the dike is nowhere observed is 
that it really never existed. The limestone walls, however, are 
not everywhere decomposed beyond recognition. In some favored 
situations the monzonite matter fits the slickensided wall as wax in 
a mould, and the metamorphic action is so slight as to be appre¬ 
ciable only for a few inches. Had the Government experts only 
visited several of the dikes with the iron belts, instead of only the 
one, they too could not have helped to find the evidences conclu¬ 
sive that the Chupadera irons are not contact deposits in any sense 
of the word. As it is, their other categorical assertions concern¬ 
ing the genesis of New Mexican ore deposits are now open to 
serious question. Keyds. 
Oil Pools in Flat Arches in Montana. A most important geo¬ 
logical discovery recently made in Montana was the striking of oil 
