This difference depends upon the heat-conducting power of the rocks. Moreover, in the neighbourhood of an extinct volcano, through gneiss, it has been observed that the increase of a degree is only attained at every 125 feet.
Añadiré, además, que en las proximidades de un volcán apagado, y a través del gneis, se ha observado que la elevación de la temperatura era sólo de 1° por cada 125 pies.