The heat of the sun never penetrates more than 100 feet below the earth's surf ace, and at which depth the thermometer remains at 62° F. throughout the year, whether in the Arctic or tropical zones, says the "Mining World. " Downward from 100 feet, every 66 feet means a gain of 1° of heat, and at 10,000 feet it is estimated water would boil. At a distance of 30 miles all rocks in the crust would melt. It is improbable that the same rate of temperature increase continues to the earth's center, because that would give a heat vastly beyond conception. We know that in the borings of mines heat increases with depth.
The large number of hot lakes and geyser springs, numerous volcanoes in various parts of the world, are certain indications that a vast amount of heat still lies beneath the earth's surface. Not long ago supposition was that the interior of the earth was a vast ocean of molten rock, surrounded by a cold crust, but later knowledge of physical laws has led to the belief that the earth is more rigid than a globe of steel of similar size. If the earth was a globe of molten material surrounded by a thin crust, it would be continually pulled out of shape by the attraction of the moon, and great tidal waves would be created on the surface by the surging molten mass within. While the interior of the earth is rigid, an exceedingly high temperature nevertheless prevails, and it continues in a solid state only by intense pressure.
If it were possible to strip off the cold exterior crust, the solid and heated interior would instantly become molten. It is most probable that there are local pools and reservoirs of molten materials in and under the crust. Whenever there is a local disturbance caused by the shrinkage of the crust and a release of pressure, we may suppose that pools of liquid rock are formed. Through fissures in the earth's crust such molten masses have in the past ages been forced up and spread over the surface. The lava beds of Arizona, New Mexico and other western points, the Palisades of the Hudson River, the Giant's Causeway in Ireland and numerous other occurrences, are remnants of ancient lava flows.