It is unnecessary that I should pursue the historical sketch further. Those who wish to know the views of writers of the Middle Ages will find them recorded by Sir Charles Lyell.[6] The long controversy carried on during the latter part of the eighteenth century between "Neptunists," led by Werner on the one side, and "Vulcanists," led by Hutton and Playfair on the other, regarding the origin of such rocks as granite and basalt, was finally brought to a close by the triumph of the "Vulcanists," who demonstrated that such rocks are the result of igneous fusion; and that in the cases of basalt and its congeners, they are being extruded from volcanic vents at the present day. The general principles for the classification of rocks as recognised in modern science may be regarded as having been finally established by James Hutton, of Edinburgh, in his _Theory of the Earth_,[7] while they were illustrated and defended by Professor Playfair in his work entitled, _Illustrations of the Huttonian Theory of the Earth_,[8] although other observers, such as Desmarest, Collini, and Guettard, had in other countries come to very clear views on this subject. The following are some of the more important works on the phenomena of volcanoes and earthquakes published during the present century:--[9] 1. Poulett Scrope, F.R.S., _Considerations on Volcanoes_ (1825). This work is dedicated to Lyell, his fellow-worker in the same department of science, and was undertaken, as he says, "in order to help to dispel that signal delusion as to the mode of action of the subtelluric forces with which the Elevation-Crater theory had mystified the geological world." The second edition was published in 1872. 2. This was followed by the admirable work, _On the Extinct Volcanoes of Central France_, published in 1826 (2nd edition, 1858), and is one of the most complete monographs on a special volcanic district ever written. 3. Dr. Samuel Hibbert, _History of the Extinct Volcanoes of the Basin of Neuwied on the Lower Rhine_ (1832). Dr. Hibbert's work is one of remarkable merit, if we consider the time at which it was written. For not only does it give a clear and detailed account of the volcanic phenomena of the Eifel and the Lower Rhine, but it anticipates the principles upon which modern writers account for the formation of river valleys and other physical features; and in working out the physical history of the Rhine valley below Mainz, and its connection with the extinct volcanoes which are found on both banks of that river, he has taken very much the same line of reasoning which was some years afterwards adopted by Sir A. Ramsay when dealing with the same subject. It does not appear that the latter writer was aware of Dr. Hibbert's treatise.