VOLCANOES AND IGNEOUS INTRUSIONS 



317 



3H 



-Volcanic neck in the Mt. Taylor region, New 

 Mexico. (Photo. D. W. Johnson.) 



located in the necks 



of volcanoes, the 



brecciated rock of 



which is called "blue 



ground " and con- 



tains the gems. 



These latter necks 



have a diameter of 



300 to 1000 feet. 

 Age of Volcanoes 



in the United States. 



— In regions of ex- 

 tinct volcanoes every 



stage in the process 



of demolition may be studied (Fig. 315 A, B), from the perfect cone, 



whose slopes have 

 as yet barely been 

 touched by erosion, 

 to that in which the 

 only evidence that a 

 volcano formerly ex- 

 isted is to be found 

 in a spot of igneous 

 rock, a few feet or a 

 few hundred feet 

 in diameter, sur- 

 rounded by sedimen- 

 tary or other rock. 

 The various stages 

 in the erosion of vol- 

 canic cones are well 

 shown in the western 

 United States, where 

 every gradation may 



Fig. 315. — Diagram A shows an active or recently extinct volcano with widespread 

 lava flows at its base. Diagram B is the same region after prolonged erosion. The ash 

 of which the cone was composed has been eroded away, leaving the volcanic neck pro- 

 truding. The lava flows have been cut by erosion into flat-topped hills or mesas. 

 In the section on the front of A the former successive positions of the streams are 

 shown, their courses having been diverted as they were filled with lava from the 

 volcano. (Modified after Davis.) 



