3 16 



PHYSICAL GEOLOGY 



the vent was so great as actually to rend the cone. Into these cracks 

 the lava was forced and cooled. It will readily be seen that a cone 

 buttressed by dikes will be greatly strengthened, and that such a 

 cone will be better able to withstand erosion than one composed 

 entirely of fragmental materials. 



Necks and Plugs. — After the upper portion of a cone has dis- 

 appeared, the neck (Fig. 312), as the compact lava or debris filling 



mm m% 



Fig. 312. — Diagram illustrating the destruction of volcanoes. (After A. Geikie.) 



the vent is called, is exposed. The neck is composed either of lava 

 or of the rocks or other fragmental materials which fell back into the 

 crater and were consolidated to form a volcanic breccia. They vary 

 in diameter from a few yards to two miles. Volcanic necks or plugs, 



when exposed by erosion, are 

 often conspicuous features of 

 the landscape. Many ex- 

 amples are to be found in 

 North America. From Mon- 

 treal one can see several hills 

 of this origin. In New 

 Mexico, Arizona, California, 

 and other western states of 

 the United States volcanic 

 necks are to be seen. They 

 are not uncommon in por- 

 tions of Europe, where they 

 are frequently the sites of 

 castles or churches (Figs. 313, 

 314). When erosion has 

 succeeded in entirely tearing 

 down a volcanic cone, it is 

 often found that the neck 

 pierced the surrounding rock 

 without the aid of a fissure or fault, and that it is independent of the 

 folds of the locks. The great diamond mines of South Africa are 



KlG. 31 ;. Volcanic neck upon which a chapel 

 has Ijeen built. Le Puy, France. 



