. The Vermilion iron-bearing district of Minnesota : with an atlas . subs have been worked out. When the subdrifts are completed,the block of ore between any two levels is honejcombed with drifts with verticalintervals of 8 feet of ore. When mining above has been completed, the removal of theore pillars on the top subs begins. The pillars are sliced away, the back is caved,and the caved ore is removed in wheelbarrows to the chutes leading to the main levelbelow. The chutes are 4 feet square and lined with 2-inch plank placed on the sand or overlying timber appears, a new slice is tak
. The Vermilion iron-bearing district of Minnesota : with an atlas . subs have been worked out. When the subdrifts are completed,the block of ore between any two levels is honejcombed with drifts with verticalintervals of 8 feet of ore. When mining above has been completed, the removal of theore pillars on the top subs begins. The pillars are sliced away, the back is caved,and the caved ore is removed in wheelbarrows to the chutes leading to the main levelbelow. The chutes are 4 feet square and lined with 2-inch plank placed on the sand or overlying timber appears, a new slice is taken off the pillar andthe back of ore is caved, as before, until finally all of the subdrifts have been workedout, when the operation of caving is continued in the block below, which in themeantime will have been honeycombed by the first or preparatory subdrifts. Below the eighth level the method of mining has been modified. What arecalled intermediate main drifts are driven through the ore at intervals of 20 feet Trans. Am. Inst. Min. Eng., Vol. XXI, 1892, p. SOUDAN FORMATION. 241 instead of 75 feet, and no subdrifts are used. The intermediate main drifts are ofthe regular size, 9-foot caps and 7-foot legs, which leaves about 10 feet of ore to becaved, instead of 7 to 8, as before. Stations are made at the shaft for each inter-mediate main level. Under this modified system the removal of each 20-foot blockwill be done as before, but the putting up of raises will be saved, and it is intendedto use cars and thus do away with wheelbarrows as far as possible. PI. X, B, shows a main drift which has begun to cave under theweight caused by the wrecking of the subdi-ifts just above. PL XI,shows the miners removing the ore from a part of the mine where thecaving has taken place. The ore mined \>j the Chandler is good hard hematite, practicall}- ashard as the Soudan ore. Subsequent to its formation (p. 233) it was frac-tured by the orogenic forces which folded the rocks, and it
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