VOLCANOES. 53 



able in assisting the observer to determine the composition 

 of distant or inaccessible volcanoes. Thus the slope of a 

 lava-cone is very gentle from 3 to 10 degrees; that of a 

 tufa-cone from 15 to 30 degrees ; of a cinder-cone from 35 

 to 45 degrees ; and that of a mixed cone usually gentle 

 beneath, but topped with a steep peak of loose and scoriace- 

 ous materials. Again, some are strictly sub-aerial that is, 

 take place on the dry land and others sub-aqueous that is, 

 operate under the waters, or but rarely manifest themselves 

 at the surface ; yet both seem to act in a similar way, and 

 to discharge similar products. Further, some are cease- 

 lessly active; others become active only at long intervals, 

 and are said to be dormant ; while others have been so 

 long dormant and shown no symptoms of activity that they 

 are regarded as extinct. Between the existing and the ex- 

 tinct there is every grade of activity, just as among the 

 extinct there is every degree of antiquity. We are thus 

 led from the active craters of Etna and Vesuvius back to 

 the extinct cones of Central France and the Rhine, which, 

 in their crateriform domes and rugged .lava-streams, still 

 retain the aspect of volcanoes ; from these back through the 

 Apennines and Alps ; from the Alps to the Pyrenees ; and 

 from the Pyrenees to the Scandinavian and older mountain- 

 ranges, which have all had the same origin, though now 

 their craters and domes are obliterated, and their outlines 

 have undergone a thousand modifications from those denud- 

 ing agencies of air and water which have operated upon 

 them during untold ages. And here the reader cannot be 

 too strongly impressed with the fact that the profiles of all 

 our existing hill-ranges are more the results of waste and 

 denudation from without, than of upheaval and accumula- 

 tion from within. When a mountain is first presented to 

 us, the natural idea is, no doubt, that of upheaval and 

 accumulation, but a little reflection will soon correct the 



