On the assumptions that thermal properties of liquid and solid magma and country rock are the same,and that cooling of the magma takes place by conduction and not by development of systematic convection cells, temperature values are calculated for several types of cooling lava sheets. Isotherms for these conditions are presented. In light of the data derived by formulation of the pattern of isotherms, the assumption that joint columns form perpendicular to isotherms in a cooled igneous body is not necessarily valid. Metamorphism of country rock at specific sites related to the cooling magma is discussed.