ROCKS, 24* 



general description of volcanoes comprising one of the mos". 

 important sections of the history of terrestrial activity and I 

 have based my statements partly on my own observations, but 

 more in their general bearing on the results yielded by the 

 labours of my old friend, Leopold von Buch, the greatest 

 geognosist of our own age, and the first who recognised the 

 intimate connection of volcanic phenomena, and their mutual 

 dependence upon one another, considered with reference to 

 their relations in space. 



Volcanic action, or the reaction of the interior of a planet 

 on its external crust and surface, was long regarded only as 

 an isolated phenomenon, and was considered solely with respect 

 to the disturbing action of the subterranean force ; and it is 

 only in recent times that greatly to the advantage of geognos- 

 tieal views based on physical analogies volcanic forces have 

 been regarded as forming new rocks, and transforming those 

 that already existed. We here arrive at the point to which I 

 have already alluded, at which a well-grounded study of the 

 activity of volcanoes, whether igneous or merely such as emit 

 gaseous exhalations, leads us, on the one hand, to the mineral- 

 ogical branch of geognosy (the science of the texture and 

 the succession of terrestrial strata), and on the other, to the 

 science of geographical forms and outlines the configuration 

 of continents and insular groups elevated above the level of 

 the sea. This extended insight into the connection of natural 

 phenomena, is the result of the philosophical direction which 

 has been so generally assumed by the more earnest study of 

 geognosy. Increased cultivation of science, and enlargement 

 of political views, alike tend to unite elements that had long 

 been divided. 



If instead of classifying rocks according to their varieties 

 of form and superposition into stratified and unstratified, 

 schistose and compact, normal and abnormal, we investigate 

 those phenomena of formation and transformation, which are 

 Btill going on before our eyes, we shall find that rocks admit 

 of being arranged according to four modes of origin. 



Rocks of eruption, which have issued from the interior of 

 the earth, either in a state of fusion from volcanic action, or 

 in a more or less soft, viscous condition, from plutonic action. 



Sedimentary rocks, which have been precipitated and depo- 

 sited on the earth's surface from a fluid, in which the uiost 



