In this communication, we demonstrate that the intrusion temperature of about 600 degrees C for the Cottage Grove dike provided by Stewart et al. [Stewart, A.K., Massey, M., Padgett, P.L., Rimmer, S.M., Hower, J.C., 2005. Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, Illinois. International Journal of Coal Geology 63, 58-67] is questionable, because their computation is based on an unrealistic heat flow model and the estimated contact temperature is derived from a geothermometer that is not calibrated for contact metamorphism. In order to obtain an accurate and reliable intrusion temperature based on the contact temperature of the intrusion, a reliable geothermometer in high temperature ranges should be used to calculate the peak temperature. (C) 2007 Elsevier B.V. All rights reserved.