LACCOLITHIC MOUNTAINS 
31 
Seeming Incongruity of the Bysmalith. A third structural as¬ 
pect of dome mountains is the one designated by Prof. J. P. 
Illings as “Plug” mountains, or Bysmaliths, with Mount 
Holmes at the southern end of the Gallatin Range, in Yellowstone 
National Park, as the type. The theoretical conception of this 
structure is graphically represented below (figure 4) : 
Neither in the preliminary description, nor in the more mature 
account are the necessary details given to prove that this form 
of bulging intrusion is to be properly classed with laccoliths rather 
than with volcanic plugs. More recent visits to the Yellowstone 
region discloses facts that point to the essential structures of 
Holmes Mountain as not referable to intrusive phenomena but to 
diastrophic movement. However, under present consideration 
these circumstances are not pertinent. The main thing to be noted 
is the fact that profound faulting is associated with the phenome¬ 
non. Its direct bearings upon typical laccolithic intrusion is 
referred to at length elsewhere. 
Until recently the conception of a bysmalith existed only as a 
hypothetical possibility. Only with the disclosure of the Ortiz 
structures does it become an object of actual observation. Insofar 
as it relates to a special modification of the laccolith is Iddings’ 
definition a valid one. Professor Hobbs' attempt to widen the 
meaning of the term imparts a totally distinct significance, and 
16 Journal of Geology, Vol. VI, p. 704, 1898. 
17 Mon. U. S. Geol. Surv., Vol. XXX. 
18 Earth Features and their Meaning, p. 442, New York, 1912. 
