284 UNSTRATIFIED ROCKS 



Devil's Tower), a magnificent shaft of columnar phonolite, which 

 rises 700 feet above a platform of horizontal strata. This tower 

 is the remnant of a laccolith from which the covering strata, and 

 probably much of the igneous core, have been eroded away. In 

 southern Utah the Henry Mountains are a group of laccoliths 

 from which several thousand feet of overlying strata have been 

 removed and the cores deeply dissected. In the Elk Mountains 

 of Colorado are some enormous laccolithic masses. 



Fig. 128. — Little Sun Dance Hill, South Dakota. (U. S. G. S.) 



Bosses are rounded or irregular masses of intrusive rock, which 

 may be only a few feet or several miles in diameter. Their ex- 

 posure on the surface is due to the removal of overlying strata, 

 and their prominence as hills is caused by their greater resistance 

 to denudation than that of the enclosing stratified rocks. Some 

 bosses are believed to be the subterranean reservoirs which once 

 supplied volcanoes ; but this can rarely be proved, because when 

 the boss is exposed by denudation, the volcanic neck has been 

 swept away. However this may be, many bosses probably never 

 communicated with the surface by any vent. Veins, dikes, sheets, 

 and various irregular protrusions are frequently given off from 

 bosses. Bosses are made up of the granitoid, compact or por- 

 phyritic members of various rock groups : granite, diorite, basalt, 

 and gabbro are especially common, and the coarseness of the 



