52 



PHYSICAL GEOLOGY 



FIG. 31. Diagram of a young volcanic 

 mountain. 



surface from a few hundred yards to a number of miles, and in 

 many cases increase in size downwards, their sides cutting ir- 

 regularly across the surrounding rocks. In New England, 



eastern Canada, and 

 other regions, many gran- 

 ite stocks form hills be- 

 cause of the more rapid 

 erosion of the less-re- 

 sistant inclosing rocks. 

 Stocks differ from batho- 

 liths chiefly in being very 

 much smaller, and from 

 laccoliths and bysmaliths 

 particularly in their rela- 

 tions to the surrounding 

 rocks. Igneous rock oc- 

 curring as laccoliths, bys- 

 maliths, batholiths, and 

 stocks is. not n beds, has 

 no cleavage, and its crys- 

 tals are without system- 

 atic arrangement. Accordingly, it is said to have a massive 

 structure. 



Volcanoes are, geologically speaking, short lived. When 

 the volcanic forces die 

 away or find relief 

 through other vents, no 

 further additions are 

 made to the cone of a 

 volcano (Fig. 31), which 

 in time is worn away 

 by the agents of erosion. 

 Long after it has disap- 

 peared, the resistant 

 rock formed by the slow solidification of the lava which re- 

 mained in the tube leading down from the crater may remain 



FIG. 32. Diagram showing a volcanic 

 neck and several mesas (p. 167) resulting 

 from the long continued erosion of a vol- 

 canic mountain similar to that shown in 

 Figure 31. 



FIG. 33. Volcanic neck near Adair, south- 

 eastern Colorado. A cylindrical mass of 

 basalt occupies the throat of an extinct 

 volcano, and is surrounded by an accumu- 

 lation of talus. (U.S. Geol. Sure.) 



