In 1964-1965 at the mine No. 3 of Slantsy Combine the resistance of layers of the industrial oil shale stack to mechanical destruction was determined. Resistance (strength) was determined in three ways: 1) by the method of pushing with the help of the POC device (strength coefficient f); 2) by the method of cutting from the open face with the help of the DKS-3 device (strength index A); 3) by the energy indicators of the Ural-33 cutting machine (strength index Avr)....