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. of recoverable oil. In the rich Mahogany ledge are .l26.billion barrels of which 80 billion barrels are recoverable. The proved recoverable. petroleum reserves of the United States amount to 27 billion barrels. , , The Bureau of Mines has made real contribution to development of these natural resources. It was early realized that the key to obtain- ing fuels .from shale on a basis competitive with fuels from petroleum was the development of low-cost mining methods. . The Bureau developed the presently used room-and-pillar method of mining where pillars 60 feet square areJeft to support the roof. Pillars are spaced 60 feet apart in rows with alternate rows staggered in one direction. This results in rooms 60 feet by 180 feet in one direction and in the other direction 60 feet ~y the depth to which the mine is opened. The distance from the. roof ~o the floor of the "Mahogany ledge" is 72 feet in the present mine but increases in the direction of mining. It is necessary to mine this depth of shale in two steps. First a heading is driven at the top of the bed, and drilled to break a face 39-feet by 60-feet wide and usually pulls to a depth of approximately 20 feet. About 2,400 tons. are broken per round. After a heading has been advanced the required distance, the second, or benching. operation takes place. Holes are drilled vertically downward in the floor about 34 feet and for a width of the drift, and blasted. By these two operations the full bed of shale (73 feet) is mined. .. In order to effect low mining costs the shale must be mined on a large tonnage basis using .a minimum of manpower. Since existing mining equipment was not adequate, the Bureau designed and built special machines for compressing air. drilling, scaling, and loading. These machines have proven satisfactory and have permitted phenomenal tonnages and low costs. )' Another major accomplishment of the Bureau is design of the Gas- Combustion retort. Many retorts have been used for retorting shale in this country or abroad where shale has long been a source of oil. All these r,etorts deliver hot products,gaseous and solid. The gases have to be cooled to condense oil vapors and produce liquid oil. Such cooling requires large quantities of water. In commercial operation the retorts would be located close to the mines in order to reduce material-handling cost'and obtain maximum space for spent-shale disposal. The present mine is nearly 3,000 feet above the Colorado River. Obviously pumping large quantities of water to such heights would be most expensive even if the required quantities were available. <Ii Because of pumping cost and limited water supply the Bureau de~ veloped a Gas-Combustion retort requiring no cooling water. It delivers all products at cool temperatures because the cold raw shale is used to cool the off-coming gases and the cold in-coming gas which is used to sweep the oil vapors from the retort as they are formed, as well as fur.nish part of the heat, cools the hot spent shale so that it leaves the retort cold. N.one of the streams e.ither into or out of the retort will have temperatures much over 2000F., yet in the retort te~peraturesup to I, ,WOOF, may be attained by both gases and solids. The Gas-Combustion retort having been fully tested on a small pilot-plant scale, a large pilot- plant unit is now under test. This unit can continually procellS over 15Q tons of shale per day and will yield design data for construction of large commercial-scale retorts. . -19-