46 PHYSICAL DESCRIPTION OP THE UNIVERSE. 



seizes the predominant character of mountain chains, 

 whether parallel, or transverse and intersecting, and whether 

 belonging to the same or to different epochs and systems of 

 elevation ; it examines the mean height of continents above 

 the present surface of the sea, or the position of the centre 

 of gravity of their volume; the relation of the highest 

 summits of the great chains to the general line of their 

 crests, to the vicinity of the sea, and to the mineral charac- 

 ter of the rocks of which they consist. It depicts to us the 

 eruptive rocks as active principles of movement, traversing, 

 uplifting, and inclining at various angles, the passive sedi- 

 mentary rocks . it considers volcanoes either as isolated, or 

 ranged in single or in double series, and extending their 

 sphere of action to various distances, either by means of 

 long narrow bands of erupted rocks, or by earthquakes 

 operating in circles which widen or contract in the course of 

 centuries. It describes the strife of the liquid element with 

 the firm land ; it shews the features which are common to 

 all great rivers in the upper and in the lower portion of 

 their course, and how they become subject to bifurcation. 

 It characterises rivers either as breaking their way through 

 great mountain chains, or following, for a time, a course 

 parallel to them, either close to their foot or at a considerable 

 distance, according to the influence which the elevation of 

 the mountain system may have exercised on the neighbouring 

 plains. It is only the general results of comparative oro- 

 graphy and hydrography which belong to the science whose 

 proper limits I am endeavouring to trace, and not the enu- 

 meration of our loftiest mountains, active volcanoes, or 

 rivers with the extent of their watershed and the number of 

 their tributaries. All these details belong to geography 



