Two cgs units of heat are in common use, and thermal conductivity is commonly given in either of two units. Units of cal/cm sec ° C are used here; to convert to watts/cm ° C, the values given in the tables should be multiplied by 4.184. A unit sometimes encountered in engineering work is the British thermal unit per square foot per hour for a temperature gradient of 1 °F per inch; this unit equals 3.446 × 10−4 cal/cm sec ° C.
Rocks are not homogeneous, and their constituent crystals predominantly are of low symmetry. The fundamental relationship given above is not valid for infinitesimal elements in this case. If no large-scale departures from isotropy exist, however, a single conductivity, determined from a sufficiently large sample, suffices to give the relationship between average heat flow and average thermal gradient. Such