There were numerous volcanoes in the geological past; therefore, since volcanic eruptions are generally attended by earthquake shocks, it follows that during that remote past the earth has been violently shaken by earthquakes. Indeed, if we assume, as we believe to be the case, that the cause of earthquakes is correctly to be traced to an originally heated globe which is gradually cooling, it follows that the earth was necessarily subject to great earthquakes almost from the time when it began to cool. But to establish as a fact the occurrence of an earthquake at so remote a time in the earth's history is far more difficult than to detect the occurrence of a volcano at that time. While the earthquake shocks may produce fissures in the earth's crust, and may be accompanied by great changes of level, yet the great time that has elapsed between such occurrences and the present would permit the various geological agencies that are at work either to cover these fissures completely, or completely to remove by erosion, or in other similar ways, the rocks in which they occurred. It is different in the case of a volcano; for the volcanic craters are in many cases still left standing, and then there are the voluminous sheets of lava that have spread over great areas of the earth, as well as numerous volcanic cones. Besides, there are thousands of square miles of surface that have been covered, often to great depths, by deposits of volcanic dust thrown out at one time or another from the craters of the then active volcanoes. I am sure you will acknowledge that any force capable of causing great cracks or fissures in the earth's crust, must, while doing this, have produced violent shakings of the earth. Great cracks or fissures are to be found in the rocks of all the geological formations. These are a record of the earthquakes that must have attended these convulsions. And there is plenty of evidence to show that the earth's crust has been torn into these fissures in places deep down below the present surface; for, by the action of water, many of these portions have been uncovered so that these great cracks or fissures which have been afterwards filled with a molten rock that has hardened can be seen in the great dikes that still remain.