The reader will perceive that all of the above theories agree in one respect, viz; in the agency of _elastic bodies_, as steam and gases, produced by the decomposition of substances; and that the same substances are supposed to be employed, though not precisely in the same manner, nor in the same condition. Further Geological and Chemical experiments may, hereafter, settle the question between these theories. They are all scientific in their principles, and fully competent to the object, and it is not impossible but that they may all be true in part or in whole, acting separately in some instances, and combined in others.
Under the agency of either of them the products would be the same. Dr. Kennedy has made experiments on the composition of volcanic products, and found, Silex, 51--Alumina, 19--Lime, 9.5--Oxide of iron, 14.5--Soda, 4--Muriatic acid, 1--in 100 parts.
As it regards the extent of volcanic action and earthquakes, the two first theories agree best with actual appearances. They would lead us to conclude that volcanic action was necessarily more extensive in the earlier ages of the world than now. Because, every action would oxidize the crust of the earth deeper, and increase the superincumbent strata, and render the access of water and air more difficult. _This is found to be the fact by observation in different countries._
In some parts of France, in which kingdom there is not, and has not been for the last two thousand years, any active volcano, _there are ranges of extinct volcanos, in which may be counted from seventy to one hundred craters_. They are so close their bases touch in many instances. The same fact is observed along the Rhine, and in Hungary, and other countries. _See Ure's New Syst. Geol. and Bakewell's Geology._
Mr. Ure reckons up two hundred and five _active_ volcanos at this time. One hundred and seven in islands, and ninety-eight on continents.
It is very evident that the _seat_ of volcanic action is vastly below the surface of the earth. The _extent_ of country which is shaken by the effort of the gases to escape, will prove this. The agitations have been felt over all Europe, and even _across the Atlantic_!
Again: If the action were not situated far below the surface of the earth, the mountains, which only serve as _chimnies_, and which have been formed by the action of the volcanos, _would sink in_. This has been the case in a few instances. This will appear more clearly if we consider the _amount_ of matter ejected. Did it not come from an immeasurable distance beneath, the accumulated mass at the surface of the earth would break down the _substrata_ which lie over the immense caverns formed by the ejection. The _dimensions_ of those caverns, situated under the volcanic mountains, are far greater than one would suppose at first consideration. _The internal caverns must be as large as the ejected masses, which came out of them._ Ætna is known to have thrown out matter sufficient to form twenty such mountains as it is. It is strongly probable that the whole island of Sicily is of volcanic origin. Humboldt says the dome-shaped craters of volcanos rise from six hundred to eighteen thousand feet in height. He considers the _whole mountainous district of Quito as one immense volcano_.