Twenty-four granitic core samples from the Illinois deep drill holes yield thermal conductivity values in the range 29.3--37.6 mW/cm ¿C, wet bulk densities in the range 2.608--2.960 g/cm3, and water accessible porosity in the range 0.12--0.78%. Thermal conductivity values vary with modal quartz content of the rocks at all depths, and both parameters decrease linearly with depth between 673 and 1016 m in drill hole UPH 3. Below 1070 m there is no systematic variation in modal quartz content with depth and hence no systematic variation in thermal conductivity. Similarly, no systematic variations in modal quartz or physical properties with depth are observed in drill hole UPH 2.