The average heat flow of the North American craton and the Canadian Shield differs by 0.33 µcal per cm2 sec for acceptable stations. A large glacial correction, used by some researchers to explain the phenomena, is rejected due to the lack of supporting data from a test hole that is 2.9 km deep, and the correspondingly low heat flow values from shields in nonglaciated regions near the equator. The contributors to the variance are (1) a sampling error due to the lack of the low heat Superior province basement beneath craton stations; (2) the heat-producing capability of the sedimentary cover; (3) the variable but thicker craton heat-producing crystalline crust. Corrections for these variables yield a revised heat flow variance of 0.05 µcal per cm2 sec. A mantle heat flow of 0.68 µcal per cm2 sec is calculated for craton and shield areas.