NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in the .pdf document. The thermal activity in Iceland is grouped into two groups, the low-temperature and the high-temperature activity, in accordance with the temperature at the base of the circulation systems. The low-temperature activity includes those thermal areas where this temperature is below 150 [degrees]C. The high-temperature activity includes areas with a higher temperature. The concentration of free CO2 in the spring gases and the concentration of dissolved SiO2 in the thermal water furnish information about the base temperature. The isotope ratios D/H and O18/O16 are indicative of recharge areas and the general pattern of flow. Temperature conditions in near surface layers are studied by the electric resistivity methods. The total heat transported by the low-temperature activity is estimated at 0.2 to [...] cal/sec and by the high-temperature activity at 0.3 to [...] cal/sec. Temperature conditions in 3 non-thermal wells in Iceland are studied and corrected for various effects, mainly the heavy erosion during the Pleistocene. The outward conduction of heat in Iceland appears to be of the order of 3 to [...] cal/sec [...] which is 2.5 to 4 times the normal. The abnormal conduction flow appears to be the main source of energy for the low-temperature activity whereas large volcanic intrusives of recent origin appear to supply the high temperature activity. The abnormal conduction flow ray be caused partially by large magmatic intrusives in the upper 10 to 20 km under Iceland.