

396 STRUCTURAL AND FIELD GEOLOGY 
till sooner or later the symmetry of the original cone dis- 
appears. Undermined and breached in every direction, the 
mountain eventually loses the distinctive aspects of a growing 
voleano. After a prolonged period all that may be left pro- 
jecting above the general level of the land may be the core 
or plug of igneous rock which cooled and consolidated in the 
funnel or pipe of eruption. Volcanoes, then, may be looked 
upon as typical “accumulation mountains,” in which, while 
they are still growing, internal structure and external form 
coincide. No sooner are they extinct, however, than they 
begin to lose their characteristic shape, and a time at last 
arrives when there ceases to be any correspondence between 
structure and configuration. 
But volcanoes are not the only examples of “accumulation 
mountains.” Materials are heaped up at the earth’s surface 
by other than subterranean action. Thus dunes owe 
their origin to the action of wind, while ,szoraznes are built 
up by glaciers. True, neither dunes nor moraines attain the 
dimensions usually indicated by the term mountain. The 
term, however, is rather popular than scientific, and in a 
scientific classification must be taken to include not only 
the loftiest elevations, but inconsiderable hills and monticles as 
well. For our classification is based essentially on geological 
structure and origin, and takes no note of the relative 
_ dimensions of hills and mountains. Amongst “accumulation 
mountains,” therefore, we must arrange dunes, moraines, 
and all other hills which are due to the heaping-up of 
materials at the surface by natural causes. Many of these 
epigenetic hills are doubtless of insignificant size. Dunes 
and moraines, for example, do not very often exceed 100 
feet, but now and again the former attain heights of 400 
to 600 feet, while the latter not infrequently reach greater 
heights, sometimes forming hills over 1000 feet high. 
(6) Deformation Mountains.— Under this head are 
included all mountains which owe their origin to deformation 
of the earth’s crust. Three types are recognised, namely, 
Folded Mountains (due essentially to folding and crumpling 
of the crust), Dzslocation Mountains (due to fracturing and dis- 
location of the crust), and Laccolith Mountains (due to bulging- 
up of the crust over intrusive masses of igneous rock). 

