874 



PRINCIPLES OF STRATIGRAPHY 



of pressure sends forth a stream of water carrying sand and mud 

 with it. (See farther in Chapter XXJII, on Seismology.) 



Dissection of Volcanoes. When volcanoes have become 

 extinct the ordinary forces of erosion set in and progressive 

 destruction goes on. The rapidity with which this takes place 

 varies, of course, with the nature of the material, the prevailing 

 strength of the erosive forces, and with other fiictors. Many of 

 the Tertiary volcanoes of the Eifel in Germany and of the Auvergne 

 district in France are still almost perfect, while others of earHer 

 date show all stages of dissection. Of interest in this connection 

 is the Kammerbiihl near Franzensbad in northern Bohemia, which 



Fig. 238. The KamiTierl)uhl, an old 

 volcanic hill in Bohemia. 



Fig. 239. Section of the Kammer- 

 hiihl, showing the probable 

 former outline of the volcano : 

 a, metamorphic rock ; b. basaltic 

 scoriae; c, plug or neck of basalt; 

 d, stream of basalt ; e, alluvial 

 beds. 



Goethe pronounced an extinct volcano, though Werner had explained 

 its character as originating, in common with those of others of 

 similar aspect, through the combustion of a bed of coal. Goethe 

 predicted the finding of a core of volcanic rock in the center of 

 this hill were a tunnel driven into it horizontally. The excavating 

 of this tunnel in 1837 verified this prediction, while more recent 

 excavations have revealed the entire structure, showing that the 

 small lava stream on the side of the hill was connected with the 

 central plug or neck and rested on basaltic scoria. The above 

 figures show the appearance of this hill and the structure ascer- 

 tained by these excavations. (Figs. 238, 239.) 



In extensively dissected volcanoes often only the central neck 

 or plug remains, as in the case of the volcanic necks of the Mount 

 Taylor Region in New Mexico (Johnson-22 ijoj-jp^), the Leucite 

 Hills of Wyoming (Kenip-25), and many others of this type. 

 Dikes dissecting the tufa beds of old volcanoes often stand out 

 in bold relief owing to the steady removal of the easily eroded tufa 

 enclosing them. Examples of such are known from many locali- 

 ties. The geological map of the Spanish Peaks region in Colorado 

 shows excellently the numerous radiating dikes which center in the 

 old volcanic necks of that region. 



