Of course, it is impossible to say whether such substances as calcium carbide, etc., actually exist inside the earth's crust, yet, as has been pointed out, the principal condition necessary for their formation, i. e., a high temperature, existed at times long after the earth, assuming the correctness of the nebular hypothesis, was separated from the nebulous sun. There still remains to be discussed the most curious of all possible causes that have been suggested for the presence of the local heated areas at comparatively short distances below the earth's crust; namely, radio-activity. In 1896, Henri Becquerel, a Frenchman, while investigating the power of the X-rays, when passing through certain substances, to produce phosphorescence, or causing the substances to shine in the dark, made the extraordinary discovery that some of the salts of uranium possess the power of emitting a peculiar radiation closely resembling the X-rays, that is able to pass through substances opaque to ordinary light as well as to affect photographic plates. But the most extraordinary part of this discovery was that the salts of uranium apparently possess the power of giving out this radiation continuously without being exposed to the sun's rays. This peculiar property was called _radio-activity_, and was shortly afterwards found to be present in many other substances besides uranium, and notably so in two newly discovered elements known as polonium and radium. Now it has been suggested that if there existed somewhere beneath the earth's crust in these locally heated areas, large quantities of radio-active substances, these regions would at last become highly heated, and in this way likely to produce volcanoes and earthquakes. It would not, however, seem that this is probably their true cause. From what has just been said it is clear how exceedingly difficult it has become to explain the source of the earth's interior heat when the fact of the earth's original highly heated condition is denied. We are, therefore, disposed with Russell to believe, as stated in the first part of this volume, that the ultimate cause of both volcanoes and earthquakes is to be found in the gradual cooling of an originally highly heated globe, and that the greater part of the interior is still in a highly heated condition, hot enough to be melted but yet in a solid condition by reason of the great pressure to which it is subjected. CHAPTER XXXV EARTHQUAKES OF THE GEOLOGICAL PAST--CATACLYSMS