The electrical conductivities of carbonaceous slate were measured using a complex impedance spectroscopic technique at 0.5-1.5 GPa and 423-973 K in the frequency range of 10(-1) to 3.5x10(6) Hz. Experimental results indicate that the conductivities of carbonaceous slate slightly increased with increasing temperatures and pressures, respectively. At a certain temperature range, the conductivities of carbonaceous slate follow an Arrhenius relation. There are three Arrhenius relations for the conductivities of carbonaceous slate at a certain pressure. From high temperature range to low temperature range, the activation enthalpies for the conductivities of carbonaceous slate are found to be 0.02-0.03 eV, 0.05-0.06 eV, and 0.11-0.13 eV, respectively. Electron conduction is proposed to be the conduction mechanism for carbonaceous slate at high temperatures and pressures. It is suggested that the unusually high conductivities of carbonaceous slate (0.1-1 S/m) are associated to interconnected amorphous carbon. Furthermore, the electrical conductivities of carbonaceous rocks can be used to interpret the high-conductivity layers (HCLs) in the Earth's interior.