The Michigan College of Mining and Technology, in cooperation with the Calumet and Hecla Copper Company and the author, is carrying out a program of temperature measurements in the deep copper mines of Northern Michigan, extending the previous work of Agassiz and others. Temperatures are measured with mercury thermometers mounted in bakelite tubes, placed in drill holes in mine workings where the rock has been freshly exposed, special attention being given the effects of drilling, blasting, and other heat conduction considerations. Present results give as the average gradient from the surface to 5679 feet below (temp. 95.3°F), 1°F in 108.5 feet (0.0168°C/meter). The gradient is more nearly uniform than has sometimes been supposed. A preliminary attempt has been made at calculating the previous ``thermal history'' of this region. Diffusivity of specimens of the rock measures 0.0075 c.g.s., and on this basis calculations of theoretical temperature‐depth curves have been made for 25 different assumptions of previous temperature conditions, and compared with the actual curve. Results as yet are inconclusive but indicate that at least 30,000 years have elapsed since the last glacial epoch, a longer period than usually assumed.