DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, an underground, modified in situ, oil shale retort 10 located in a subterranean formation 12 of oil shale is covered with an overburden 14.
Retort 10 is elongated, upright, and generally box-shaped, with a top or dome-shaped roof 16.
Retort 10 is filled with an irregularly packed, fluid permeable, rubblized mass or bed 18 of different sized oil shale fragments including large oil shale boulders 20 and minute oil shale particles or fines 22.
Irregular, horizontal and vertical channels 24 extend throughout the bed and along the walls 26 of retort 10.
The rubblized mass is formed by first mining an access tunnel or drift 28 extending horizontally into the bottom of retort 10 and removing from 2% to 40% and preferably from 15% to 25% by volume of the oil shale from a central region of the retort to form a cavity or void space.
The removed oil shale is conveyed to the surface and retorted in an above ground retort.
The mass of oil shale surrounding the cavity is then fragmented and expanded by detonation of explosives to form the rubblized mass 18.
Conduits or pipes 30, 32 and 34 extend from the above ground level through overburden 14 into the top 16 of retort 10.
Pipes 30, 32 and 34 include ignition fuel line 30, feed gas line 32 and purge gas line 34.
The extent and rate of gas flow through lines 30, 32 and 34 are regulated and controlled by valves 36, 38 and 40, respectively.
Burners 42 are located in proximity to the top of the bed 18.
In order to commence retorting of the rubblized mass 18 of oil shale, a liquid or gaseous fuel, preferably a combustible ignition gas or fuel gas, such as recycled off gases or natural gas, is fed into retort 10 through fuel line 30 and an oxygen-containing, flame front-supporting, feed gas, such as air, is fed into retort 10 through feed gas line 32.
Burners 42 are then ignited to establish a flame front 44 horizontally across the bed 18.
If economically feasible or otherwise desirable, the rubblized mass 18 of oil shale can be preheated to a temperature slightly below the retorting temperature with the a preheating before introduction of feed gas and ignition of the flame front
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Liquid Purification 
